Navigate to https://www.onshape.com/en/education/sign-up
Fill in the information on this page and click 'Create EDU Account'
Note: Make sure you use an email that you will immediately have access to as you will need to use it to verify your account. We have not had luck with using school emails because they can't receive emails from outside of MPS so a person email account usually works best.
Fill in the information for your school and agree to the terms then click 'Create Account'
You can simply put 'FIRST Robotics' for what you plan to use Onshape for
Check the email you used to sign up in the earlier step
Open the Onshape account activation email and click the link to activate your account
You will be sent to a page where you can now create your username and password
If you are part of Riverside, fill out this form with your email address. We will use this to add you to our team in Onshape which will give you access to the document for our robot when we start the season!
FRC 3005 CAD Curriculum: https://onshape4frc.com/robochargers-curriculum#week1
This does an excellent job of explaining CAD and engineering concepts specific to FRC.
Onshape Introduction to CAD Curriculum: https://learn.onshape.com/learning-paths/introduction-to-cad
This isn't specific to FRC but is the best resource I've found for learning CAD skills.