I like to have a bit of room under the screen so I can water and fertilize etc. so around 12" to 16" from the top of the pot is where I like to be: Right around 28" total height. It makes for a compact plant that I can move around readily and still get a good yield (8 to 12 oz).
I would top, fim and supercrop and based on how the plant responds I would put it into the screen. Usually I do the SCROG a couple of weeks before I flip so I have a bunch of plant material to thread through the screen. Only one plant per screen as you will almost certainly be dealing with (harvesting) plants at different times. Plus if you have a problem it’s really a good idea to separate the plants if disease or something else causes you to need access to your plant.
Really; it’s dead easy or I wouldn’t do it. It’s a great way to turn a round dripline into a rectangular one. My grow space is 48" by 72" so I divided my SCROGs into thirds so I can do 3 plants in that space. My screens are 22" by 36" which allows for some airflow front and rear and makes room for mechanical stuff.