function onOpen() {
  try {
    const spreadsheet = SpreadsheetApp.getActive();
    const menuItems = [
      {name: 'Prepare sheet...', functionName: 'prepareSheet_'},
      {name: 'Generate step-by-step...', functionName: 'generateStepByStep_'}
    ];
    spreadsheet.addMenu('Directions', menuItems);
  } catch (e) {
    // TODO (Developer) - Handle Exception
    console.log('Failed with error: %s' + e.error);
  }
}
A special function that runs when the spreadsheet is open, used to add a
custom menu to the spreadsheet.