I've never really practiced it, they just happened because I was lucid in the last dream and remembered to keep still. However, I AM practicing now, since I got myself a DEILD alarm. So what I've been doing is taking "fake" naps, and when the alarm goes off, I stay perfectly still. The problem is, these last two nights I've woken up before the alarm, and it ends up ringing as I'm trying to go back to sleep. It's pretty fucking annoying. If it keeps happening, I'm just going to practice staying still when I wake up naturally. Anyways, once you start practicing, you'll find out whether you need to visualize, or if it happens on it's own. Everyones different! No worries though. If the conditions are right, you're going to have a lucid.