I like to use the shuttle myself but also use the needle occasionally, I know some people prefer the needle too. I am interested that you find the needle the best way to teach, I find a good way to teach the double stitch is to teach them to make chains first in two colours and then they can always see that the stitch has flipped correctly. When my students graduate to rings, I teach them to pull on the thread under the hand each time they make a stitch so that they know if it will slide correctly or not. And I use a good, thick thread such as size 10.