It wouldn't be a platformer without jumps. For most readers, this section will take up the largest chunk of time. Once you start designing a level, you'll find it's a lot of fun to lay out all the pieces, creating challenging jumps, secret paths, and dangerous encounters.
First, you'll create a generic Level scene containing all the nodes and code that is common to all levels. You can then create any number of level scenes that inherit from this master level.