Snap code, combined with an .xlsx file you use as a template, can create an Excel output file. Here are some examples.
In your output rule, use the "Add output field" block to insert whatever characters you want into a specified cell location.

A better approach is to avoid hard-coding fundamental elements you'll refer over and over, such as the name of the sheet. Create a text variable to store the sheet name, so you can change it easily if you need to:

Usually you're not writing to an individual cell, but many cells (as a row), or many rows (as a table). Here's how. The following code starts on whatever sheet is set in the "tableSheetName" variable, and at whatever row is specified in the "tableSheetStartRow" variable. From there, it creates a table starting at column H, ending at column K.

You can also delete the contents of a cell. For example, if a cell is filled with some text in the template, and you don't want it to appear, simply delete it.

Adding an image to an Excel sheet is done by specifying the image, and then defining where it will appear in a sheet. This location is defined by the upper-right and lower-left cells to be covered by the image.

Some Excel templates may have native Excel formulae defined in the template. To ensure each formula on a sheet is refreshed and calculated based on the data you've inserted, call the "tryCalculateFormula" block. This will update all those formulae, graphs, and other native Excel calculations on the one sheet you specify. Call this command as one of the last steps in your output rule, after you've inserted all your data.
For example, the block of code below gets a handle to the sheet name stored in the variable "introSheetName", and then ensures any formula/graph/etc on that sheet is updated.

For example, if you are inserting two sheets, but want to update values only on the second sheet:

The examples above are available for you to cut-and-paste into your own output rule. Just select all the example code below(double- or triple-click), copy it, open up an output rule, right-click in the empty white workspace of the rule, choose "paste JSON", and press CTRL-V to paste it.
Example Code
[{"$type":"CommentBlock","disabled":true,"x":55,"y":139,"parts":{"comment":{"value":"Only generate this file if a certain condition is met"},"next":{"$type":"SetOutputPropertyBlock","disabled":true,"parts":{"property":{"value":"enabled"},"value":{"$type":"CompareBlock","parts":{"left":{"$type":"GetFieldValueByNameBlock","parts":{"name":{"$type":"LiteralStringBlock","parts":{"text":{"value":"myLanguage"}}}}},"operator":{"value":"=="},"right":{"$type":"LiteralStringBlock","parts":{"text":{"value":"en"}}}}},"next":{"$type":"CommentBlock","parts":{"comment":{"value":"Set the filename. Make it obvious when this is not a production document."},"next":{"$type":"SetOutputPropertyBlock","parts":{"property":{"value":"targetFilename"},"value":{"$type":"StringConcatBlock","parts":{"v":2,"parts":[{"$type":"GetVariableBlock","parts":{"variable1":{"value":"quote"},"variable2":{"value":"name"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"-"}}},{"$type":"CastBlock","parts":{"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"quoteProduct"},"variable2":{"value":"id"}}},"type":{"value":"text"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"-Summary"}}}]}},"next":{"$type":"IfBlock","parts":{"v":2,"if":{"$type":"CompareBlock","parts":{"left":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"environment"}}},"operator":{"value":"!="},"right":{"$type":"LiteralStringBlock","parts":{"text":{"value":"prod"}}}}},"then":{"$type":"SetOutputPropertyBlock","parts":{"property":{"value":"targetFilename"},"value":{"$type":"StringConcatBlock","parts":{"v":2,"parts":[{"$type":"CastBlock","parts":{"value":{"$type":"GetOutputPropertyBlock","parts":{"property":{"value":"targetFilename"}}},"type":{"value":"text"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"-"}}},{"$type":"GetVariableBlock","parts":{"variable1":{"value":"environment"}}}]}}}},"next":{"$type":"CommentBlock","parts":{"comment":{"value":"initialize some variables"},"next":{"$type":"DeclareVariableBlock","parts":{"itemVar":{"value":"introSheetName"},"type":{"value":"text"},"default":{"$type":"LiteralStringBlock","parts":{"text":{"value":"Sheet1"}}},"next":{"$type":"DeclareVariableBlock","parts":{"itemVar":{"value":"tableSheetName"},"type":{"value":"text"},"default":{"$type":"LiteralStringBlock","parts":{"text":{"value":"Sheet2"}}},"next":{"$type":"DeclareVariableBlock","parts":{"itemVar":{"value":"tableSheetStartRow"},"type":{"value":"number"},"default":{"$type":"NumberBlock","parts":{"number":{"value":"5"}}},"next":{"$type":"CommentBlock","parts":{"comment":{"value":"Write one field: basic syntax"},"next":{"$type":"AddOutputFieldBlock","parts":{"$propertySettings":{"externalId":{"$type":"LiteralStringBlock","parts":{"text":{"value":"$C$5@Sheet1"}}},"value":{"$type":"LiteralStringBlock","parts":{"text":{"value":"Hello, World!"}}}},"type":"excelCell","v":"HelloWorld","next":{"$type":"CommentBlock","parts":{"comment":{"value":"Write one field: suggested syntax"},"next":{"$type":"AddOutputFieldBlock","parts":{"$propertySettings":{"externalId":{"$type":"StringConcatBlock","parts":{"v":2,"parts":[{"$type":"LiteralStringBlock","parts":{"text":{"value":"$C$5"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"@"}}},{"$type":"GetVariableBlock","parts":{"variable1":{"value":"introSheetName"}}}]}},"value":{"$type":"GetFieldValueByNameBlock","parts":{"name":{"$type":"LiteralStringBlock","parts":{"text":{"value":"Fmaterial"}}}}}},"type":"excelCell","v":"Project Name","next":{"$type":"CommentBlock","parts":{"comment":{"value":"Write many fields with a loop"},"next":{"$type":"DeclareVariableBlock","parts":{"itemVar":{"value":"ThisRow"},"type":{"value":"number"},"default":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"tableSheetStartRow"}}},"next":{"$type":"ForEachBlock","parts":{"itemVar":{"value":"item"},"arrayVar":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"configurator"},"variable2":{"value":"priceObject"},"variable3":{"value":"items"}}},"statement":{"$type":"AddOutputFieldBlock","parts":{"$propertySettings":{"externalId":{"$type":"StringConcatBlock","parts":{"v":2,"parts":[{"$type":"LiteralStringBlock","parts":{"text":{"value":"$H"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"$"}}},{"$type":"CastBlock","parts":{"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"ThisRow"}}},"type":{"value":"text"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"@"}}},{"$type":"GetVariableBlock","parts":{"variable1":{"value":"tableSheetName"}}}]}},"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"item"},"variable2":{"value":"sku"}}}},"type":"excelCell","v":"SKU","next":{"$type":"AddOutputFieldBlock","parts":{"$propertySettings":{"externalId":{"$type":"StringConcatBlock","parts":{"v":2,"parts":[{"$type":"LiteralStringBlock","parts":{"text":{"value":"$I"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"$"}}},{"$type":"CastBlock","parts":{"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"ThisRow"}}},"type":{"value":"text"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"@"}}},{"$type":"GetVariableBlock","parts":{"variable1":{"value":"tableSheetName"}}}]}},"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"item"},"variable2":{"value":"description"}}}},"type":"excelCell","v":"Desc","next":{"$type":"AddOutputFieldBlock","parts":{"$propertySettings":{"externalId":{"$type":"StringConcatBlock","parts":{"v":2,"parts":[{"$type":"LiteralStringBlock","parts":{"text":{"value":"$J"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"$"}}},{"$type":"CastBlock","parts":{"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"ThisRow"}}},"type":{"value":"text"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"@"}}},{"$type":"GetVariableBlock","parts":{"variable1":{"value":"tableSheetName"}}}]}},"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"item"},"variable2":{"value":"qty"}}}},"type":"excelCell","v":"Qty","next":{"$type":"AddOutputFieldBlock","parts":{"$propertySettings":{"externalId":{"$type":"StringConcatBlock","parts":{"v":2,"parts":[{"$type":"LiteralStringBlock","parts":{"text":{"value":"$K"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"$"}}},{"$type":"CastBlock","parts":{"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"ThisRow"}}},"type":{"value":"text"}}},{"$type":"LiteralStringBlock","parts":{"text":{"value":"@"}}},{"$type":"GetVariableBlock","parts":{"variable1":{"value":"tableSheetName"}}}]}},"value":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"item"},"variable2":{"value":"extNetPrice"}}}},"type":"excelCell","v":"Price","next":{"$type":"SetVariableBlock","parts":{"variable1":{"value":"ThisRow"},"value":{"$type":"MathOperationBlock","parts":{"v":2,"parts":[{"$type":"GetVariableBlock","parts":{"variable1":{"value":"ThisRow"}}},"+",{"$type":"NumberBlock","parts":{"number":{"value":"1"}}}]}}}}}}}}}}}},"next":{"$type":"CommentBlock","parts":{"comment":{"value":"Delete the contents of a cell "},"next":{"$type":"IfBlock","parts":{"v":2,"if":{"$type":"CompareBlock","parts":{"left":{"$type":"FormatDateBlock","parts":{"date":{"$type":"NowBlock"},"format":{"$type":"LiteralStringBlock","parts":{"text":{"value":"$EEEE"}}}}},"operator":{"value":"!="},"right":{"$type":"LiteralStringBlock","parts":{"text":{"value":"Friday"}}}}},"then":{"$type":"AddOutputFieldBlock","parts":{"$propertySettings":{"externalId":{"$type":"LiteralStringBlock","parts":{"text":{"value":"$C$7@Sheet1"}}},"deleted":{"$type":"TrueFalseBlock","parts":{"value":{"value":"true"}}}},"type":"excelCell","v":"HideFridayDealText"}},"next":{"$type":"CommentBlock","parts":{"comment":{"value":"Add an image to a field"},"next":{"$type":"CommentBlock","parts":{"comment":{"value":"Note: adjust the media block used by the filePath parameter: click it to select media from your org's resources folder."},"next":{"$type":"AddOutputFieldBlock","disabled":false,"parts":{"$propertySettings":{"name":{"$type":"LiteralStringBlock","parts":{"text":{"value":"LogoField"}}},"filePath":{"$type":"ImageMediaBlock","disabled":false,"parts":{"image":{"value":"rubik's_cube.png"}}},"source":{"$type":"LiteralStringBlock","parts":{"text":{"value":"Media"}}},"sheetName":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"introSheetName"}}},"upperLeftCell":{"$type":"LiteralStringBlock","parts":{"text":{"value":"C16"}}},"lowerRightCell":{"$type":"LiteralStringBlock","parts":{"text":{"value":"E22"}}}},"type":"excelImage","v":"CompanyLogo","next":{"$type":"CommentBlock","parts":{"comment":{"value":"Try to update formulae in the template"},"next":{"$type":"AddOutputFieldBlock","disabled":false,"parts":{"$propertySettings":{"externalId":{"$type":"GetVariableBlock","parts":{"variable1":{"value":"introSheetName"}}},"tryCalculateFormula":{"$type":"TrueFalseBlock","parts":{"value":{"value":"true"}}}},"type":"excelSheet","v":"UpdateFirstSheet"}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}]
Here's an example Excel template file, which matches the Snap code above: Excel.Output.Template.Example.xlsx