Pretty much like booking an indie, except at GEA you can't book in advance, just on the day. I'd suggest calling for your first booking, but others book by text. Call after 9am and tell the call handler that you'd like to book (insert SP's name) for (insert duration) at (insert time) for incall or outcall
Assuming she is not booked already, they will take your punting name and number, check with the SP and confirm by text including the info you need to access the flat if an incall.
Once in the flat, you pay the SP the agreed fee and proceed as normal. The SP will pay the agency their cut.