If you haven’t figured it yet:
To buy a shirt you click the gray icon, like the Men’s “M” button for adult men’s medium
Then you can click “Checkout”, then click “Checkout” again
If you haven’t done so yet, you have to fill out your info
Click “Continue” at the bottom and it should be easy from there

Enjoy your “Migratory Pattern” shirt, I know I am :D
If you’re still confused just send the website an email

Also, it asks for a discount code, you can use mine, I’m not using it: QS8YXB
Just something the website does, gets 10% off purchase :)