Tuba lesson? Ok then.
Sounds like you want to be coming from the side for sure. Which side depends on which foot you have forwards whilst you are sliding. If you are left forward, approach from the left, right forward, from the right. It is possible to go from either side and still slide it natural, but a lipslide (tails over first) is harder.
If you have any easier down features (flat downs, down boxes) around, maybe practice on those first, urban style rails are not the easiest.