Hi there! 
Basically, what’s happening here is that your body is starting to go to sleep. A lot of people who WILD report vibrations before SP. I find it to be interesting. At this stage, however, its important to try and not focus on your physical body. If you focus too much on the shaking, you’ll stay awake. During this part, I like to imagine a dreamscape, with the hopes that I’ll eventually be dreaming that I’m in that dreamscape.
Other dreamers take a completely different approach, and try to “roll over” into a dream, “sink” into their beds, or “floating” out of their bodies.
Its really all about what works best for you. Keep trying and eventually you’ll get it
The WILD FAQ article and WILD - A User Friendly Tutorial are really helpful! Take a look!