Follow the link to Eventbrite from the membership page.

Scroll down the page (or press the big obvious orange button which does this automatically)

When you reach the "Tickets" section (as shown in the image), use the blue plus (or minus) buttons to select a membership of the category you are choosing (i.e. adult, associate, or junior).

The big orange button will change to show the cost of your selection. When ready, click/tap this.

You'll be taken to a check out/payment page. Provide your name, email, and payment details