Self-adhesive fabric forms a secure bond to clothing through many washings, and does not require wet ingredients. Cut out shapes and peel off the backing, then press the fabric to the onesie. To create more professional looking cutouts, use stencils to guide your design; or get creative and make your own shapes, characters or lettering.
Stencils are a onesie decorator's best friends. Buy some letter or shape stencils and use them to trace shapes on scraps of fabric, or directly on the onesie. Use fabric paint to fill in the shapes. Spell out the baby's name or a funny phrase or saying that will make all who see it laugh.
Iron-ons are another option for onesie decorating. They are available in every imaginable style, from band logos to cartoon characters. If the mom-to-be has a favorite movie or a favorite actor, find an iron-on with a relevant quote or image that will make her smile. For newborns of sophisticated parents, consider a silhouette of Che Guevara or Dostoevsky.
For those artistically-inclined onesie decorators, fabric paint is the most versatile and free-form tool. Paint a landscape, a cartoon or an impressionist water lily. Remember that the parents are the ones who will be appreciating your handiwork the most, so consider their interests when looking for inspiration.