Arriving. Tap Arrived when you're at the pickup. Your phone's location confirms it — within about 100 m, a little more when GPS is weak. If you're too far, you'll be told how far. The rider's free waiting starts from a confirmed arrival, and waiting is paid only on one.
Rider didn't come. Once you've waited the rider's free minutes plus 5 more after a confirmed arrival, you can end the trip as "Rider didn't come". It isn't counted against you, you go straight back online, and the rider isn't charged.
Stops are steps: tap Arrived at each one and Continue when you leave. The first 3 minutes at a stop are free; after that you earn the ride type's waiting rate, up to 15 minutes a stop, on a confirmed arrival. When the rider booked a wait at a stop, the offer and the stop show it: that wait is paid in the fare, and minutes past it earn the waiting rate, up to 15.
Round trips. The offer says ROUND TRIP and how long the rider booked you to wait. At their destination, tap Arrived and wait; the booked wait is paid in the fare, and minutes past it are paid at the waiting rate, up to 30. With no wait booked, the rider's first few minutes there are free (3 on most ride types); after that you earn the waiting rate, up to 30. When the rider is back, tap Start return. If they haven't come back 15 minutes after the booked time, you can end the trip there.
Ending away from the drop. If the rider asks to get out early or the road is blocked, you can still end the trip — you'll be asked why, and the rider may be asked to confirm.