1. Click on a shirt image to get to the product.
2. Click the "Customize It" button (it's under the orange "Add To Cart" button).
3. Pick the view to work on (front or back).
4. Click the "Add Text" or the "Add Image" button depending on what you want to do (or add both if you want!).
Once you type in text or add an image, you can click the Design button above the photo to drag and drop it where you want or use the little arrow icons.
If you don't like what you've done, just click the "Revert" link at the bottom of the page to get it back to its original state.