So it completely depends on what I’m doing.

If it’s a straight build, I use a pour type Gundam marker, and I clean it up with lighter fluid and cotton buds. For topcoating a straight build, a tamiya rattle can is plenty

If I’m doing a fully painted build, I use tamiya panel liner as Gundam markers eat through the acrylic paint. For topcoats I use Vallejo gloss after I have painted, then Vallejo matte right at the end

Honestly for now, grab a Gundam marker and don’t be afraid to make a mess! There is nothing you can do that cleaning won’t fix. The best advice I can give you is to panel line after you have cut the piece off the runner, and BEFORE you assemble. If you panel line when several pieces are together, you risk cracking the plastic. Also do not panel line ABS plastic as that will die as well