Click "Build New" and select "Shirt". You can also simply find "Shirts" in the list already existing.
Look just right of the "Create New" button. There should be a title that says "Create a Shirt". Just below that, there is text that reads, "Did you use the template? If not, download it. The last three letters in the text, "download it here", are hyperlinked to the template picture. Click those words to reach the template picture, which will open itself in a new tab.