Once you're on the Customize screen (step 3) in Design & Print Online, you can add a text box using the menu on the left or, if you've selected a pre-designed template, you can click in the center of the screen to select the text box.
If all your labels will have the same information, erase the placeholder text by clicking and dragging your mouse to highlight the text, then use your keyboard to type in your new text. You can also use the garbage can icon at the top or the Delete key on your keyboard.
If each of your labels will have different information, set up the design for your whole sheet before you make changes to any one label. Then you can switch to Edit One in the Navigator near the top right and click to select the label you want to update first.
Here's a quick overview: