First you need to visit your creator's sign-up page.
If you don't have a direct URL to their sign-up page (e.g. hubermanlab.supercast.com), you can visit supercast.com and use the "Search for a podcast" field at the top right of the page. Type in the name of your creator to find their sign-up page.
From the creator's sign-up page, you can select the plan you'd like to subscribe to and then click "Subscribe Now" to check out.