I used a Cricut Explore Air 2 for this project, but any vinyl cutter should work just fine. I've added a link to the project on the Cricut Design Space so you can customize the design or the message to match your style.
Or, you can download this image file for use with other vinyl cutters or vinyl cutting services.
Different sized t-shirts will want different sized designs. Luckily, it's easy to customize the size in the Cricut Design Space. Just select the group and drag the corner until it's the size you want.
Next, select all the layers except the circle "moon", and click "Weld". This will ensure the whole design cuts as it's laid out on the design board.
Do a test cut on a sheet of paper or card stock and hold it up in front of your shirt to be sure you like the sizing and placement.
After you're sure you're happy with the layout, load your iron-on vinyl into your vinyl cutter liner side down. This means shiny-side down, or upside-down, most of the time. Double check to be sure you have this right! This stuff is expensive.
Set your vinyl cutter to the "Iron On" setting and cut.
Peel the unused vinyl carefully off the liner. It works best to pull horizontally, close to your work table (instead of upwards toward the ceiling).
Take a utility knife or other small sharp tool and carefully "weed" the design, pulling out any vinyl that shouldn't be there and leaving just your perfect design on the liner.
Line up the design on your t-shirt. Read the directions that came with the iron-on vinyl for best results. Preheat the fabric, then iron on both sides and let the design cool completely before removing the liner.