First things first, you're going to have to create the mug design you want. There are tons of premade mug patterns in Design Space that are ready to go. If you want something more custom, just use the correctly sized layout to create your own design.
You can use any of the Cricut digital cutting machines to cut out your design. Make sure to select "Infusible Ink Transfer Paper" as the material you're cutting.
Remove the cut transfer paper and weed out all the excess pieces.
Wrap the design around a lint-free mug, matching up the transfer tape end tabs inside of handle. You may need to reposition a few times until the pattern is perfectly lined up. If you want to really secure the transfer paper to the mug, use a few strips of the heat resistant tape along the top, side, and bottom.
Pre-heat the Cricut Mug Press by pressing the power button and waiting until it beeps. Insert the mug into the opening. Make sure the design is completely covered inside of the press. You may need to readjust the handle as you're pressing down on the lever.
Once the machine cycles through all the flashing lights above the power button, it'll beep to let you know your mug is done. Lift the lever to release the mug and carefully pull it out by touching only the handle. The body of the mug will be extremely hot so make sure you only touch the handle and it's placed on a heat-safe surface.
Allow the mug to cool completely before removing the heat resistant tape and the Infusible Ink Transfer Paper design. Revealing the finished look is my absolute favorite part!
Now you've got a awesome custom-made mug to enjoy for a really long time!