Layers in Pro

How do I Layer my design in Sketchup Pro?

In what sense exactly?
You can use Tags, formerly called Layers, to help toggle visibility. But I’m not sure if that’s what you are asking about.

The ambiguity @monospaced alludes to is exactly why the name Layers was changed to Tags a few versions ago. Layout has 2D layers much the same as other 2D drawing apps, but that’s not equivalent to what SketchUp tags do in 3D.

If you are looking for ways to stack objects in layers (like a layer cake) in SketchUp, that’s a function of modeling in 3D space.

Tags are ways to control visibility, line types, and to categorize similar objects. There’s more depth to it, but that’s a basic explanation.

You can add new tags in the tag palette by clicking the + at the top of the palette. Then select the object and, in the Entity Info palette, change the assigned tag using the dropdown menu. There are other ways about it, too.