I just enrolled in my first craftsy class two days ago called "the Costume box" by Anne Weil. It was a fantastic series of explanatory videos to make your own costumes for boys and girls, with a very cheerful teacher who gives tons of tips along the way. I can warmly recommend it, she has plenty of easy to do projects made for the busy moms that we all are :-)
One of the projects I completely fell for was the knight costume, I cooked up a quick little visual tutorial, mainly to make myself some pointers to do it again in the future, but for more detailed explanations you can watch the class for only 10USD, totally worth it.
The idea is to use a piece of soft cardboard (ex: cereal box) dimensions required: 40x9 cm
And cut the following shape out of it:
Then we sew it to a piece of fabric and trim
tip: glue the fabric to the cardboard first so that it stays in place
Then sew the elastic