1. you don't need to sleep beforehand, but if you sleep for 5 hours or so, your body is primed to re-enter REM (dreaming) sleep...

2. if going from the waking state (with no previous sleep that day), it can take up to 2 hours.
if you have slept for awhile (especially more than 4 or 5 hours), it is possible to wake up and re-enter a lucid dream only seconds later (had this happen numerous times)

3. some people experience no SP, some experience it locally, some feel it all at once all over their body...it is slightly different for everyone, but always completely harmless and nothing to be alarmed about

welcome, and good luck in your journey