To lock a sheet tab in Google Sheets, simply right-click on the desired sheet tab and select "Protect sheet" from the context menu. Alternatively, navigate to the "Data" menu at the top of the interface and choose "Protect sheet." Then, set the permissions for the sheet, specifying whether collaborators can edit, comment, or view it. Click "Set permissions" to apply the protection. You can verify the protection by attempting to edit the protected sheet, where you'll find that modifications are restricted.