You only need to repeat the mantra for a bit of time and then fall asleep like you normally do. When I used to MILD I would only repeat the mantra for 5-8 minutes maximum.

As far as WILD and SP/REM Atonia being a scary prospect, it's really only scary if you expect it to be. Personally a very large majority of my many experiences in SP/REM Atonia have been fun and interesting because I knew what was happening wasn't real and I actually enjoyed and looked forward to future experiences, so I'm sure that's influenced how the experiences turned out quite a bit. When I was first trying WILD back in 2008 I was a bit scared and apprehensive from what I'd heard about SP too, but it's really nothing to fret over. It's really a rather cool phenomenon to experience imo.