Start with a green shirt. The closest to his coloring the better, but I'll be honest, it was hard to find this shirt.

***a gleam in your eyeball is a nice touch***

For the mouth, take your fabric/felt and fold it in half. On one half draw a half mouth shape. Then cut. When you unfold it, you'll have an equal sized boat-like shape.

The next part takes the longest, but it adds character to the costume. Cut teeth out for your monster mouth (tiny teeth for the bottom row and bigger teeth for the upper. Adding a little point is ideal because, well he's a monster!)

And then attach the mouth to the shirt as well.

And there you have it! A creative, goofy Mike Wazowski costume! You don't have to add his horns, but I did using gray felt and hot gluing it to a hard headband.) 

Let me know what you're being for Halloween!