We'll start with relatively short and practical, and for the sake of simplicity I'll assume you're twisting to the left. If you're a right twister, you can just reverse things.
Don't try to twist.
Do the following steps in order:
1) Lie on your back on the floor, arms by ears. Push the back of your hands into the floor, and lift your hips up just enough that your butt comes off the floor. This is your layout takeoff position.
From here, while keeping your arms straight, bring them forward/out in front of you and down to your sides until your palms are on the floor by your sides. Hips should still be lifted, body should still keep tension; other than arms and shoulders, nothing in your body should move while your arms are coming down. Memorize those exact positions, that exact body shape, those exact muscle movements; that's your layout, and it wouldn't hurt to do this drill every practice before you do layouts.
2) Warm up a good, strong, high back layout (ideally either on a trampoline or into a pit; if that's no an option, do this wherever you find a layout the easiest)
3) Back layout, but after you set, your left arm goes straight out to the side on the way down; right arm can just stay up towards the ceiling.
If you keep your body straight and tight, moving the left arm out to the side will cause you to start twisting.
Once you start twisting, bring both arms towards your body to accelerate. They can come down to your sides, or they can come to your chest, whatever makes most sense to you; all that's important is that they come in closer to your body, and your body stays straight and tight.
4) Focus on what you see throughout the skill. It's not enough just to have your eyes open; you should actively focus on what you see. This is an important habit to build in all flipping and twisting skills, but a back layout 1/1 is where it starts to make a huge difference in the quality and consistency of the skill.
More technical, but still practical:
PAY ATTENTION TO WHAT YOU SEE. Every single time you do any skill involving rotation, take a moment to remember exactly what you saw, and think about what this tells you. In a full specifically, you should see the following things in order:
1) The wall in front of you. You should feel your feet completely off the floor, toes pointed, while still looking at the wall in front of you. Then the left arm goes out to the side to initiate the twist
2) The landing beneath you. After your left arm goes out to initiate the twist, after about the first 1/4 turn, you should be able to look to your left and see the trampoline/pit/mat/floor.
3) The landing beneath you, still. As your feet flip over top and your twist continues, you should be able to watch the landing the entire time, without ever losing sight of it. If there is any point after the first 1/4 where you can't see the landing, you did something incorrectly; try to figure out what that is. It might be a mistake in timing, or it might be a mistake in head or body position; either way, remembering what you saw during the skill can probably help you figure out the solution.
Nerdy physics stuff:
Everything there is to know about flipping and twisting boils down to understanding conservation of angular momentum. I'll do my best to explain it here, but I'm not exactly a physics professor, so apologies in advance if I do it poorly:
On takeoff, we leave the floor with both linear momentum and angular momentum.
Linear momentum determines our trajectory, and once we are in the air there is absolutely nothing we can do to alter that momentum or that trajectory. It can only be altered by some outside object or force (gravity, the floor, a bar, a coach spotting you, etc). Gravity is just such an outside force; however, because its effects are constant, our trajectory remains unalterable for our purposes; there's nothing we can do in the air that will cause us to go higher or farther; all of that is determined and set in stone once our feet leave the floor.
Angular momentum is rotating power. Again, once we are in the air, there is absolutely nothing we can do to alter our overall amount of rotating power, nor can we alter its overall direction, BUT there is some clever manipulation we can do with this momentum to give us a surprising amount of control over how we rotate.
If we can pull our mass closer to our axis of rotation -- in other words, if we tuck -- we can make more efficient use of our angular momentum, and thus rotate faster. When we tuck, we're not increasing the amount of rotating power; we're just increasing our efficiency in using it.
If we can create a lateral asymmetry in the body's orientation -- in other words, if we can get our body to tilt slightly off-axis while flipping -- then we can cause some of that flipping power to be "borrowed" as twisting power. Again, neither the amount of angular momentum nor its overall direction changes; we're just changing how we use that power.
Once we've started twisting, we can do so more efficiently by pulling our arms in closer to the body; this works for exactly the same reason that tucking increases your flipping speed; we've decreased the rotating radius, and thus increased our efficiency. And we can bring the arms out wider if we need to slow down.
So our goal in any twisting skill should be to take off efficiently with the appropriate amount of angular momentum, then get our body to tilt off-axis after leaving the floor. There are a number of ways to accomplish this, but the simplest is to just bring one arm out to the side; as the arm rotates to the left side, the rest of the body will get an equal-but-opposite tilt to the right side, causing us to twist to our left. When we see the landing, we bring the arms out to slow down the twist and (ideally) eliminate the off-axis tilt, giving us a safe, controlled, consistent landing.
