As others described I build a jig on a piece of plate. Simply put I lay out the part on the plate in chalk, weld round stock pegs in the corners to bend around, angle iron keepers on the outside to prevent rebound when bending the next corner. Make a tool to bring the stock you are bending around the rod tight. Basically, a bar with a hole to fit over the round stock pegs and concave surface under the bar to push the stock you are bending tightly around the peg. You need something to hold onto the end when you start so that it doesn't pull away from the starting point when you make your first bend. make your jig close to the edge of the plate and you can use a good vice-grip.
Also, drill a hole through your plate in the middle so you can bolt it directly to a heavy welding table. Preferable to have your table attached to the floor or it will move when bending.

I have a bending tool , one thing to remember is the size rod you're bending .
I bend 1/4 inch rod , and three bends adds 1 1/2 inch to the length . So for a 12x12 trap , you start out with a length of 1/4 rod at 46 1/2 inches .
You have to have a good mark to go by , as it you don't get it right , one side will be longer than 12 inches .
The starting point needs to be at the center of the first bend , that started as a blunt end .
It's like said above practice and practice .
90 Degree bends .

Dad is a retired electrician.
He can do things with a bender and a piece of conduit that will make you think it is some kind of magic trick

Not as simple as it seems to those of us who don't know

Wolf you should be able to find something on youtube, even if you search bending conduit that will get you headed down the right rabbit hole

Here's where I would start to determine the free length of the rod (identifying the material consumption or your radius) and then as mentioned- fixture with stops for consistency.

That math doesn't add up to me. 4- 12 inch sides would be 48 inches. If you are adding 1.5 , you shouldn't be at 46.5?

I'm talking a hand held torch....like a plumber would have in their tool bag. Map gas (yellow colored tank/bottle) burns hotter than regular propane, and it wouldn't take long at all to heat that small of stock to red hot.......a few seconds or so.

Read it a couple times , it will come out at 48 inches.
46 1/2 plus 1 1/2 is 48 .

I read it wrong. I thought you were saying to add an inch and half to account for the bends. As in 48 add 1.5 for the bends.

My bad.