Watch the Tap Quest online guitar lesson by Jennifer Batten from Rock Sauce For Lead Guitar
I find it extremely helpful to have my right thumb touching the top of the neck as a guide when I tap. That seems to be something students need to be shown over and over again if the technique is foreign. I've also seen some people plant their upper palm on the upper horn of the guitar body as a guide instead, so experiment and see what's more comfortable for you. Be aware that the right hand has two jobs when you're tapping. It needs to make notes sound, and it also becomes a pick if it pulls off to another note. So you don't just lift your finger off the string, you slightly push off.