Planning Your Keyboard
The point of this step is for you to have a vision of what you want to make, so you can easily execute on your idea.
Sketch up what the layout of the switches should be, where the Raspberry Pi Pico should go, etc. This could be on paper or in a 3D modeling software.
Make it a weird shape, or add a cool feature, or silkscreen! The point is that you know what you want to make, and you can change this along the way.
Next Steps
Once you have a plan, head to Designing the PCB — this is the big one.