To make the first half of the doublestitch you MUST slide the stitch formed by the ball thread along the thread that comes from the shuttle without snarling the two threads. As you raise your middle finger, this stitch will slide down the thread toward your thumb. If you snarl the threads you will simply end up with a square knot and the shuttle thread will not be able to be pulled freely through the stitch. This will stop you from closing rings. Study this picture closely... the straight horizontal thread is coming from the shuttle, while the thread forming the "bubble" is coming from the ball thread.

This shows the bubble sliding down the shuttle thread as you keep raising the middle finger and eventually this stitch will be held between your thumb and the side of your index finger. Move the bubble along slowly and make sure it stays over the shuttle thread and not under it.

If you slide this stitch down between thumb and forefinger without snarling it, and then do the second half of the stitch in the exact same manner without snarling it, your stitch will be able to move along the shuttle thread. You'll be able to pull the shuttle thread through the stitch both frontwards and backwards. In effect, the stitch is formed AROUND the shuttle thread allowing the shuttle thread to be pulled through it from either end. The most difficult thing to understand about this stitch is that even if it snarls, it will look as though you did it right - but believe me, you did not. As you learn this stitch, it is wise to test each one by sliding it slightly back and forth along the shuttle thread...just a little to be certain it hasn't snarled. Once you master this stitch - you'll be tatting!