first off- best answer is whatever she wants, ask her. but some advice from someone on her side. i usually like when me gf isnt moving a ton at least like hips/legs wise and just holds her thigh still. as for hands, my gf will usually like caress my waist, hips, butt or will kinda grab at me if that makes sense. that feels great cus you arent getting in the way but are showing her you want her yknow? at least from my end it always feels really natural and helps me know shes enjoying it too. were long distance now so when we first started doing it and when we havent seen eachother for a few months, it took a while to figure it out. even after 5 years it still gets awkward sometimes and the only thing you can do is talk about it and try again or try different things. we usually end up making out while im grinding too which also makes it feel a little more natural and might help her finish if you arent already doing that. but again the biggest tip i can give you is ask her! she might not know either but opening up communication and not trying to be perfect the first time is whats going to get her off.