After selecting Extensions > Khan2GC > Setup > Launch Setup, click on the first section, Spreadsheet.
Before starting the initial build process, it’s important to understand that any data in the spreadsheet will be destroyed in the process. Furthermore, while the process is running, the spreadsheet may seem unresponsive. The status bar at the bottom of the setup sidebar will show Build Complete when the process is complete. Do not try interact with the spreadsheet until the process is complete. Click the Build button to start the process.
If your spreadsheet should freeze and you receive a message from your browser saying that the page is unresponsive, refresh the page. Even if you refresh the page, the build process will continue to work in the background. Note that after refreshing, it still may take several minutes for the spreadsheet to load and/or become responsive again. When it does, wait until only the Settings sheet appears at the bottom and your spreadsheet is responsive again before relaunching the setup sidebar and continuing to the next section.