This big baby is strapped into her high chair and ready for a good meal. To achieve the full effect, it's best to go with a store-bought setup like this one since fashioning your own high chair would be tricky. You can find this costume online at www.anniescostumes.com and at other retailers.
If you don't want to spring for the premade costume, you can go as a baby simply by dressing in pajamas (with feet, if you can find them!), a bonnet, and a bib. Costume shops stock plenty of fun baby accessories like oversized pacifiers and bottles that will add to your look!