Practical order of operations:
When booking: request wheelchair assistance in the booking itself (the WCHC code — meaning you cannot walk at all — is the one you want, not WCHR). Book an aisle seat with a movable armrest. Avoid the emergency exit row; you are not allowed there.
Your own chair: it goes in the hold, free of charge, and does not count against baggage. Remove your cushion and take it into the cabin — it is the one thing you cannot manage without if the chair is delayed. Photograph the chair at the gate. Take off anything loose (side guards, footplate) and put a tag with your name and phone on the frame.
Transfer: you go on an aisle chair at the gate, and boarding is before everyone else, deplaning after everyone else. Budget 20 extra minutes each end. Ask that your chair be brought to the aircraft door rather than the carousel — say it clearly at check-in and again at the gate.
Bladder: aircraft toilets are effectively unusable. On a 3 hour flight I taper fluids about 2 hours before, catheterise right before boarding and immediately after landing. Carry supplies in your cabin bag, never checked.
Get the assistance staff member's name. The single biggest difference between a smooth trip and a bad one is one accountable person rather than a series of shrugs.