Ok the methods he said above me are good, 1 and 2 are sort of like the same thing lol but anyways you can do a couple of things so that the plant is held down to get that horizontal effect. You can Pull them under the screen, weave them, tie them/zip tie to the screen, basically anything that will keep the plant secured to train it. Sometimes it depends on your screen, wether its made of chicken fence, wire, string, construction fence stuff, ect. So think about what you're going to have to deal with when you harvest. ( I have a screen of 2 long strings that I can un hook and take apart real easy)
Also I recommend you "almost" fill your screen before you flip ( in other words once your screen is almost full, about 6" from the sides, flip to 12/12) some will disagree but I've found that the flowering stretch gives you a good amount of vertical growth so you can get good sized buds. instead of a bunch of 6" buds you can have a bunch of 10" buds, so to say. Remember everything under the screen will not get light and you should prune away anyway so you want to maximize your yield and maximize your light. I use a 600w so I usually fill my screen all the way up and then flip and I get really nice growth above the screen while still keeping the canopy relativly even so I get the most out of my 600w penetration. This all depends on the light you use so always base when to flip on the power / penetration of your lighting.
Happy scrog'ing!