When you complete and pay for the application you are given a link to then book an appointment at a UKVCAS centre. It will list the closest centres to you from your postcode. Some "core" appointments are free (I think 6 centres) but others you have to pay. We paid £60 for my wife's ILR appointment which was during working hours. I think it's £120 for afterhours or weekend appointments.
The dates were all mostly booked up at the centres I checked within driving distance to us so we had to wait three weeks for a suitable appointment