Alright so you have two separate problems: Learning CAD yourself, and encouraging others to do so.
Let’s start with the easier problem: teaching yourself.
I have no experience with onshape, I’m assuming there’s plenty of good general tutorials out there for the basics of using the program. If you haven’t already, setup your keyboard shortcuts, do some quick searching for one and get the basics of the program down backwards and forwards.
Next comes the FRC specific stuff. The first thing I can say is to use a design library. Save yourself the effort. It’s easier to make people want to CAD if they get to spend mortise doing the fun stuff!
As has been stated throughout this thread, the way you become a good FRC designer, is quite simply to design a lot of robots. The best way to get started is to try and copy other teams. Get yourself comfortable with designing each of the 5 most commonly used mechanisms in a way they could be manufactured by your team (use COTS parts when it makes sense): 1) A tank drive, 2) an arm, 3) an elevator, 4) a ball intake, and 5) a shooter. To do this, you must study other team’s designs. Read Chief Delphi threads, and look through CAD models. If you are able to design each of these in the offseason, congrats! You are now capable of building the major subsystems of a robot for any game the GDC could reasonably throw your way!
The next problem for the FRC designer is systems integration. This sometimes can be harder to practice well without building a full robot to test how everything works together. That doesn’t mean it can’t be practiced. Here, 2D sketches (I’m assuming something similar exists in onshape…) are your best friend. Also, design some full robots out. You may not be able to build a real one to make sure everything works together, but you can still make sure everything fits and looks reasonable. Furthermore, designing a robot for a past year requires that you think of game strategy, which is inseparable from good robot design.
For more help on FRC specific design, check through Spectrum’s recommended CAD resources. (Yes, most of them are solid works design tutorials. That does not mean that they don’t have a lot of good information which you can translate into Onshape)
Ok. Second problem–how do you get others to want to do the same? This can be hard, especially in the offseason. Well, firstly you do what you can to try and help them get bitten by the robotics bug–send them cool robot designs and FRC videos until they can’t help themselves! Failing that, you could try and have some sort of design competition between the students, or make a list of things which need to be designed for the team (like a driver station, robot cart, shelving/storate, etc) to show that their work has immediate value for the team.