My nephew Barrett asked for a Captain America costume, and I was thrilled to make it! I made it a simple t-shirt project. Using XL t-shirts for scraps, I got one blue t-shirt in his size and added the white star and the red and white stripes to make it look just like Captain America. Then I used this tutorial to add muscles to the shirt! I haven't taken pics of him in it yet, but he was thrilled and his older brother was jealous, so there may be more in my future!
I just use the biggest star I can get from the basic Photoshop shapes on a normal piece of paper for an applique pattern
four inch stripes because that fit neatly around the shirt. He was a youth small.