If you want to schedule it to run weekly instead of hourly, all you need to do is to turn off the automatic backup so the default scheduler will not run it too frequently and set up an weekly recurring iCal event in one of your calendar.
The alert should be a message to warn you (like 15 minutes before starting the backup) and the actual backup script. The following AppleScript will trigger the Time Machine backup. To save it, open up the AppleScript Editor, paste it and save wherever you want to.
do shell script "tmutil startbackup"
When setting up the iCal event, add an alert saying ‘Run Script’ and choose the saved script.