A set of useful Google Sheets functions for Mystery Hunt.
Go to Code.gs and use GitHub's raw file feature to make it easy to copy and paste all of the code.
Got to your Google sheet, select the Tools menu and then select Script Editor.
Copy and paste all of the code in there, eliminating the example function they started you with. Edit or add to this code in that editor if you feel like. Press save button.
Now formulas like "=caesarShift(F2,I2)" should work in your spreadsheet.