Offerings are recorded in the 'Church Windows' system.
In addition to Messiah-specific instructions included with the counting materials, a you-tube video is available here: Church Windows Donations entry.
Note that you may also swap if you discover that you have a conflict (or you may delete). After clicking on "Swap" you will be given a list of counters who have signed up for other dates that you might swap with. Select one and they will be sent an email asking if they would be willing to swap with you.
For questions or hands-on instruction contact Dan Byers.