For our Thing 1 and Thing 2 kid costumes, I started with 2 red T-shirts and 2 sheets white felt (which I had). For the pants, my older son already had all royal blue spiderman pajama pants (tiny logo was covered by T-shirt ) and for my little one, I found a pair of royal blue PJ pants at Kmart on sale.
I used a bowl to draw a circle on both pieces of felt, and cut those out. Then by using an image to refer to, freehanded “Thing 1” and “Thing 2” with permanent marker. Hot glued both onto each shirts.
For the hair, I bought 2 blue foam visors at hobby lobby 0.99 cents each and 1 blue sheet of foam paper (same aisle). On a piece of paper I freehanded spike shapes for hair, cut it out and used it to trace onto big foam sheet. I traced 2 of these, cut them out, then flipped the visor upside down and hot-glued the pieces to each visor.
The costumes came out great, better than expected, and were done in no time.