I made Daisy and her cousin matching onesies for their birthdays a couple weeks ago. Here's an instructional if you'd like to do the same kind of project!
Onesie or t-shirt
Fabric for applique
Thread and Sewing machine
1. Cut out a square of fabric and interfacing for your applique. Follow the interfacing instructions for ironing it together
2. After you bond the fabric to the interfacing, cut out the image for your applique.
3. Measure and cut the bonding strips based on the size of your applique:
4. Lay them down between the onesie and applique:
5. Iron on the applique to the onesie.
6. Sew around the edges with a zig zag stitch set wide with close stitches.
7. Iron again to finish it up. I added bows to mine and made matching tutus as well. But that's basically it!