You find the muscles for sure first and how to activate it (the muscle is on the gooch

, you can feel it tighten when you flex it and expand when you relax it), you do sessions where you alternate between reverse (relaxing) and normal (flexing) for a couple seconds each and work yourself up from that. Don't overdo it though, it's a muscle. Plenty tutorials online.
The movement for reverse kegels was much trickier for me than normal. While normal feels a little bit like holding yourself from pissing, reverse is the opposite, it feels similar to pissing/farting without doing it. I couldn't do it with it flexing my abs or while breathing at the beginning.
Once you got the hang of it practice during different positions or even fapping and you'll see how that works