My oldest son loved it so much he asked me to make one for him to and I couldn't very well make him one without making our other son one. I've done two onesies now and two t-shirts.
I used Heat N Bond Ultra to iron them on. I have seen a lot of people sew around their design, however I am not much of a seamstress so I am hoping ironing it will work well enough. If that is not the case over time I guess I will be asking my mom for some help sewing around them!
For anyone wondering exactly how I cut my fabric I used a brand new blade and mat. I ironed on the Heat N Bond following the directions on the bag and put it on the mat fabric face up. I put my blade at 4, speed at 2, and pressure at 5. Then I followed the rest of the Heat N Bond instructions to iron them on to the shirt. Simple and Fun!