Just like the boy onesie, I got my the template from the Silhouette store and designed the size/layout for it on my Silhouette. Of course, if you don't have to use a Silhouette, you can use an editing program you'd like, or even just create one in a word document.
Once it's created, you'll want to print out a test copy, to make sure that everything fits nicely on your onesie. You'll also want to make sure that your material is pre-washed, that way the material won't shrink AFTER you've already applied the design.
Next, you'll just want to print the design on your Heat Transfer Paper (make sure that your design in printed as a mirror image!!!)
Once everything looks good, it's time to get out the iron!!
Hope you can find something fun to make for your little one too!