Hello! Adding text to any product at Cafe Press is done by uploading an image. You just need to use a good imaging program (like Photo Shop) and make an image that is just text. Start with a plain white background (or transparent if you want the text to work on a color background, like a black t-shirt). Make sure the image will be very high dots per inch - like 300 dpi. Make the image as large as Cafe Press allows. They have guidelines for image sizes, depending on if you want it on a shirt, a mug, a mouse pad, etc.
Then just write your text. Think about how it will look on a t-shirt and use as much of the space as you can, and make the letters plenty big so the writing can be read from a distance. Then upload the image to Cafe Press and select what products you want it to appear on.