Definitely start slow. If you just go for it before bed, you’re probably waking up feeling stiff/sick/hurting. Definitely make sure you get hydrated and stay that way for a little bit cause any massage if it’s not part of your daily routine is gonna release all sorts of shit into your body and your muscles might rebel, you might feel hungover etc.
I’d start off with shorter sessions and wait to start really digging in to muscles till you’re sort of used to it. My first week with a massage gun was basically recovering from the first day cause I thought I was ready.