This is exactly why I hate WILD guides . Everyone is different and their bodies work differently. Yes, it’s true, lying on your back etc. will make SP more likely, but that has little or nothing to do with WILD. Only 3-4 of my LD’s have happened from SP, whereas I’ve had over 40 WILD’s. You just need to go to sleep staying aware. And by aware I do not mean awake. You can’t dream if you’re awake.
If you do still want to go the SP route, do not do it when you first go to bed. You should do it 4-6 hours after first going to sleep. It’ll happen much faster, you’ll get in the dream quicker as you go into REM almost immediately and you’ll probably skip the SP stage altogether.
Above all, do not worry about SP, even if you’re trying to get to it. It’ll happen when it happens and you’ll know it. If you keep going “hey, I think my hand is getting numb” or whatever, you’re concentrating on your body which will prevent it from falling asleep.
I wrote a little longer about this in an article here: