1 nap

Overall, you just have to ride it out until they slowly start waking later and later. Re: timing of nap, I'd say, stretch them as long as you think is reasonable(e.g. 9:30-10am) and do a calming activity in the late afternoon to break up the second wake window and keep them until bed time. For us, during the transition phase to 1 nap (would be awake at 12noon the latest), a walk at 4pm worked a treat. LO would rest and relax but not fall asleep.

For my boy we tend to somewhat ignore him in the morning so he gets the idea he isn't supposed to be up yet. He gets a nappy change and bottle if he is being grumpy but its all done in near darkness and then we get back into bed (he has a junior bed so can move around his room on his own and come in to us but we stay in bed. Most days he will have his nap about 12-2 (just after his lunch). But sometimes he gets an earlier nap if he is super tired and will usually end up having 2 short naps instead of 1 long one e.g. 45min at 10am, 45min at 3pm, bedtime at 7pm.