First off, I do all vertical panels in a small rolling chair.
For the higher vertical sections, usually about 2-3 5.5" pad widths down from the window on a regular sedan, BMW 3 series for example, I use it with the pad at 12, left hand on the head and right holding the handle, trigger locked to "ON". Put pressure on the head by angling your body so your left shoulder is closer to the car than your right shoulder, thus putting pressure on the polisher via your whole upper body.
For the lower sections, I flip the polisher so the head is at 6, then use my right hand on the head and left hand holding the polisher handle, trigger locked to "ON" once again. Similar body position... right shoulder slightly closer to the car than left shoulder, but for these sections I'll use the inner thigh of my right leg to push my right elbow and provide pressure to the head of the polisher.
Angles obviously vary from about 11-12-1 at top and 7-6-5 at bottom but you get the idea. I also sometimes vary the hands on the head of the polisher when doing bottom sections, usually 50/50 depending on which direction I'm going in, but for the top sections I think I use my left hand on the head of the polisher a good 95% of the time.
One bit of advice I have is to put a lot more pressure when you're doing side panels because you're losing a lot of pressure due to gravity when doing vertical as opposed to horizontal panels. Took me a while to figure it out years ago but it's definitely something to consider.