Go to https://www.canva.com and create a free account by clicking "Sign up." If you already have an account, click "Log in" and enter your Canva credentials.
You have two options to starting a design: 1) Create a design from scratch or 2) Find pre-built templates.
Either option will give you a variety of options for design type and size. Choose what is appropriate for your project.
See the following guides for help on editing your Canva design:
*NOTE: Canva auto-saves as you work.
From the top right of the screen, you can either Share or Download you Canva design.
Find excellent, detailed help for the creating process at https://support.canva.com