Shortening sleeves is really easy, so easy, that I have done it several time since. This is my simple way of shortening sleeves.
Turn your sleeve under and hem. You have a few options with the hemming. You can serge the edge of the fabric before you turn it under to hem, making a traditional hem not necessary. If you are working on a knit fabric, it won't unravel, so you can turn it under and hem without worrying it will start unraveling from the other side.
I don't pin the hem on my daughter's sleeves before I start sewing, because it gives me the freedom to trouble shoot as I go if I need to. When hemming on knits, find the stretch stitch on your machine, or you can also use a narrow zig-zag. The biggest thing to remember is to let your machine guide the fabric through. If you push or pull on the fabric, since it is knit, it will stretch out and look funny.
Just a few minutes later, you have a shirt that is summer ready! I am loving the sun already!