In fact sleep paralysis just occurs naturally every time you go to sleep you don't "do" it, just to clear this out.
Attempting to WILD and thus entering SP before the first sleep cycle (a.k.a. when you first go to bed) is nearly impossible. Better do it after a WBTB, 4-6 hours after sleep, (experiment to find out the best time for you) and good luck!

EDIT: The reason for this is that it takes too long for the first REM to kick in, plus it's duration is super short. So we take advantage of the later cycles.