If you use crayons, Crayola seem to be the best because they have a strong color. Make sure you back the cotton with freezer paper as a stabilizer. Also, plain solid cotton is the best. White on white fabric does not take the color evenly.
You iron off the wax by layering the block between paper towels. I try to put brown packing paper down so I don't mess up the ironing board. Once the wax is off you can tell if the color is strong enough. If not, color over it again and iron off the wax until you have the right color saturation. If you wash it with Deft or Woolite, it should hold up.
When using color pencils, I recommend using a fabric medium instead of water.
Why don't you make a small practice piece with both methods and run it through the washer a few times?