First, cut a design from an Infusible Ink sheet. A few things to keep in mind:
Place the sheet liner side down on the mat.
Mirror your image before cutting. You can see how to do that here.
Be sure that your hands are clean and dry before handling Infusible Ink.
You will want to locate your cut so that you have enough of the sheet to cover your entire mouse pad and the design will be in the center.
I have so many more tips and tricks over on my guide to Infusible Ink if you have never worked with it before.