Flat 5's aren't too bad but they are a little scary your first time. Can you practice on a trampoline or something? that is the best way to do it. Also, is this your first inverted or off axis trick? I actually learned backflips and back 180's first, which helped with my mental game. If you're not a flipper though, this might be easier.
When I throw a flat 5, I sometimes like to carve into the jump just a bit. I take a mellow carve to the left and as I pop, I flatten my skis or even transition a little to my other edge (Right edge) At the same time, I take my left elbow and jab it straight behind me and look over my left sholder. (Sometimes it helps to think of siiting in the back seat a little too.) From there, it's all about staying tight in the air and spoting my landing, which you can actually see for a good portion of the rotation. A 5 is the most natural rotation with this trick so if you stay tight, you should come around to it. Also, grabbing it can help your rotation as well. I usually Safety or Japan with my right hand.
When first learning these, try to find a smaller, poppy jump. A super poppy, 10-20 foot table would probably do the job. You don't want to go huge on your first one and a poppy lip helps set your rotation and there are less consequences if you don't get all the way around.
Well, that's about it. The key, as with any trick, is commitment. You gotta win the mental game first. Good luck.