Amateurs don't use tripods
Just put it on a desk/table/tv... And mix it in with POV.
Once shooting a threesome one girl wanted to hold my camera while I was banging the other one. Now that footage looks really really amateur.
Shooting in HD with low light settings helps to get that amateur look again. But yeah downgrading HD is another possibility.
But amateur isn't just about the light conditions, it's about the angles, about the movement of the camera, being half off cam, a couple having fun, or a punter fucking a bored hooker, etc.