I have used the fabric crayons - sorry, can't recall the brand. With those, the image is colored on paper and then transferred onto fabric with an iron. I found the transfer made the colors really soft and not vibrant at all.
Then I worked with regular Crayola crayons and used them to color directly on the fabric. If I wanted to blend colors, I had to put white crayon down first. The excess wax was ironed off with paper towels.
I also have a set of fabric markers (not Sharpie). They have a sharp point on one side and a brush end on the other. They also need to be set with the iron.
It's a lot of fun - makes me feel like a kid.