How do I make a 2d platformer?

I'd like to create a 2D platformer in S&box, but I'm having trouble doing so. Can someone provide me with step-by-step instructions on how to set things up? Every time I try, either the gravity is in the wrong direction or the collision doesn't work, and the player phases through the ground.

this is a good place to start if you're talking about 2d with sprites.
https://www.youtube.com/watch?v=1OI4kRBjaXM

Yes but that doesnt help me with the 2d platformer physics

you can either use built in physics (rigid bodies and hull stats) or create your own

How do I make my own?

c#