If you are using an agent, you can call them up and they will handle the admin. If you are doing it yourself, you can fill in some of the initial forms online or go to the Admeasurer's office and fill it in there. (The latest guides have the addresses / details for Colon / Panama City)
To do list once you have your date:
1. Arrange lines and tyre fenders
2. Check the engine (as it will need to run for over 12 hours at various rev's to get you through). If nearing the amount of hours for an oil change or fuel filter change, it may be prudent to do it before you transit.
3. Cleats – check all your cleats and their backing plates (if they have any). Maybe add a few more (larger) washers etc?
4. Obstructions – consider the access required to the cleats for the line handlers and for the skipper to view what is going on around him. Remove weather cloths, awnings, BBQ's, outboard engines, dinghy's etc that might get in the way, obstruct visibility or snag when rafting
5. Clean the hull and prop – might get another .5 of a knot (every little helps)
6. Weight - Don't fill up with too much fuel, water and stores before you go through if you don't absolutely have to
7. Fuel – Ensure you are filled up with sufficient fuel to get you though the whole journey
8. Stock up – Don't forget to feed your crew
9. Send the webcam link to friends and family