This is my buddy, Jack. His is three and boy, through and through. I made him a superhero cape for Christmas last year and he wears it all the time. He decided that he wanted to be Superman for Halloween this year and rather than spending a ton on a cheap costume, I wanted to make him a shirt that he could keep wearing after Halloween was over.
I starting by drawing a design on paper and cutting it out. I cut red and yellow felt into the shield design first and then pinned the design to the red felt. I tried using an X-acto knife to cut out the cutouts, but then realized that it was easier to just use super sharp sissors.
After it was cut out, I used Tacky Glue to glue the two pieces together. I don't know if that was really necessary since I was going to sew them together anyway, but it made it not slip around at all and I didn't have to pin it.
The last step was just sewing it to the shirt. Easy peasy.