Upload the SVG file to the software for your Cricut machine. You can see how to do that here.
You can then cut your design from your heat transfer vinyl. Be sure to mirror your cut and put the material face down on the mat.
Once your design is cut, weed away all of the excess material. Weeding tools make this job easier.
Then place your design on your onesie and press for the recommended time and temperature. All heat transfer vinyl brands are different so you will want to follow the instructions that come with what you are using. You can also see our general guide to heat transfer vinyl here.
Once pressed, remove the transfer sheet and enjoy your new onesie!