When I attempted my first scrog I had some issues trying find out what to do. I did find tidbits of TRULY helpful info out there but doing a few yourself is the best way to understand.
To pass on the helpful tidbits...topping for a "4-way" LST seemed to work best. Let the plant grow up to the screen and through it a few inches. Then take your tops and move them to the next square on your scrog. The next square can be any direction you choose, whatever fills your canopy most ideally. The side branches that grow off your main stems are used the same way to fill in the net.
In my experience, which is not a ton, you will want to flip to flower when the canopy is 80-85% full for an indica and 60-70% full for a sativa. (Roughly). 1 week before flip you will want to clear everything, and I mean everything, that is not touching the net and receiving light.
If done correctly there is no need for a 2nd net. Most pictures you see with 2 nets are NOT a scrog. They are simply trellis used for support.