Some people diaper adults when they're standing up. This is sometimes easier than when they're laying down. They might need to be holding onto a grab bar or a second person. This can work for pull-ups too.
Otherwise you roll them to one side so that you can spread out the diaper and then roll them back and it is under them. You grab hold of bit of it between their legs and pull it into place.
For pull up ups you get them on as far up as you can go while they are laying down. Then you roll them to one side and pull them up as far as you can over their back end. Roll them again to the other side and repeat.
Repeat as often as needed.
If they're able to sit on the toilet, they should be in a routine to go on a very regular schedule. Keep the schedule.