Boo Monsters Inc Costume Diy . So i decided to post a blog specifically on how i made the monster's inc boo costume. Baby’s first halloween costume should be one to remember. Monster's Inc Boo Costume … Halloween costume monster, Monster inc from www.pinterest.com I cut arm circles into the fabric, i think i may have made them a little to big but i was winging it, it ended up working out fine. Sew all along the sides (technically 3 sides only need closing, since it’s folded), but leave an opening so that you can turn the right side out. Discover the fantastic tutorials here so you can create your own.