Getting set up
First make sure you have a copy of the productdelivery module from svn. The crontab files are in the files/cron/ subdirectory.
Testing your change
The best way to test is to use the role login (eg ffxbld) on stage.mozilla.org by connecting via a slave. You can verify your change will work as expected, but beware of any existing cron jobs you might fight with. They are running on a different host (upload-cron.private.scl3.mozilla.com, accessible by BuildVPN for RelEngers but not role accounts), which accesses the same storage.
Review and landing your change
After preparing a patch for review, vetting it for any private information that may be shown in the diff, and getting review, land the change in svn in the usual way
svn up svn diff # and confirm your change svn ci -m "Bug X, <description>, r=<someone>"
Changes take 15-30 minutes to deploy.