Simply find the shirt that you want, select your size, and where the option exists the colour of your shirt and whether you want the optional backprint, and hit the add to cart button.
You can view the contents of your current shopping cart or move on to the checkout at any time by using the small cart icon at the top of the page.
Should you wish to continue shopping, just carry on.
Any further items you add to the cart will appear in the same cart in the same way.