A lot of people use pvc, but I've found it's cheaper to build something out of wood. I used some basic string this round and it works pretty well, I'm having difficulty with it but it's my fault. If you don't tie the string as tightly as you can then the plants will just move the damn string
Doesn't have to be string; however you need something gentle enough to not snap or break branches, yet firm enough to hold them in place.
Design is simple enough and can be tailored to pretty much any grow/light footprint. Just make a wood frame, stick some hooks in it, and tie string into the hooks to form a "grid".
Put them up as soon as you've made your first top, allowing it to grow into 4 main branches with LST. Let those 4 main branches hit your screen and start tucking until 75% full if you're limited by space (flower stretch), 100% full if you have lots of room, light, and airflow to work with. Put the screen about 6-8 inches above the plant, you want them to hit the screen fairly quickly but also want the 6-8 inches of extra growth as this will end up being the support for the canopy.
One thing I will say about SCROG is that it's one of the best methods for getting the maximum potential out of pretty much any light, however I've also found that you really have to know your strain with this method. If you can't tell exactly how much the plant will or will not stretch, how much they do or don't veg, yield, etc, then your results will be somewhat of a crapshoot. Then again, I suppose this is true of pretty much any method lol
I started with seeds, all of which I had no experience with and was essentially growing in the dark with them and in a single plant per light scrog that can make it difficult to hit max yield. Some strains did much better than others when it came to filling the screens, but some strains have me wishing I put more plants under the light. My Cluster Bomb looks like it's going to throw off some of the biggest nugs I've ever seen in my life, however it fills half of the 4x4 screen I built for it. All I had to go by was the breeders description (which I usually take with a grain of salt for this reason) and it said it was perfect for a SCROG. However, it is so focused on building massive buds that it didn't branch nearly as much as the others I've got. My yields under that light will suffer, because if I had the experience with this strain I would have known to throw 4 of that strain under that light instead of just one.
Don't get me wrong, I'll pretty much SCROG exclusively to maximize yields with my plant count, but you definitely need to have a decent knowledge of how the strain is going to grow to have things 100% dialed in. Once you get that going though you can pretty much push the limits of whatever space you're in from that point on.
Hope that helps.
