Go to https://www.canva.com/nonprofits/ and read through the offer.
It is a fairly straightforward form where you'll need to upload some sort of proof of your organization's nonprofit/charitable status. Typically a government form with your legal name, address, and charity number.
I've signed three organizations up for this and got rejected every time. All I did was the appeal process, and the organization got approved.
This page has more detailed guidance: https://www.canva.com/help/canva-for-nonprofits/
Every step, every guide, with new guides added frequently.