What to wear: For the Mandalorian (aka Din Djarin), you need a Mandalorian suit complete with a mask (but we won't tell if you take it off sometimes). For Grogu, you need the classic Yoda ears and a brown robe.
How to act: This is a particularly good costume for brothers with a big age gap. As Din, you're very protective of Grogu, while Grogu is a mischief maker (who wants to eat all the Halloween candy).