Study the multiplayer tutorial. If you're going to try to make a multiplayer game without understanding how all of the multiplayer tutorials work, it will be very difficult to get anywhere. If you have a good grasp of the tutorials, you should be able to apply them to a simple platformer. Unfortunately, anything to do with multiplayer isn't going to be simple, and its hard to just take the example and change it if you aren't familiar with how it works.
If you have gone through the entire tutorial, what have you tried so far? Here is a quick checklist off the top of my head. You could probably get more help if there was something specific you are stuck on and could ask about.
Create the base game.
Join a game/room/session via signalling service. Enable host events if first to join, else enable peer events.
Host - Create, assign, and sync player objects as self/peers join. Receive inputs from players and apply to correct player objects.
Peers - Send inputs to host. Apply same inputs to relevant assigned player object for local input prediction.