(Two fingers have been omitted in this diagram. The fingertips of these two fingers (left ring and the pinkie) are inside of the ring holding the bottom of the ring down, touching the palm.)
The second hitch of the double knot is tatted as follows:
a) The shuttle thread drapes in front of the ring.
b) Bring the pick of the shuttle in from behind the ring towards you. Draw the shuttle completely through the ring thread to the front side, over the shuttle thread.
c) Bring the left middle finger, placing the fingertip on the palm of the left hand between the thumb and ring finger.
d) Pull shuttle thread to the right until the hitch transfers over to the ring thread.
It should look like this diagram.
e) Bring the left middle finger up and under the ring thread. Pull the hitch up and to the left, pulling the second hitch taut and next to the first hitch tatted.
f) Pull the shuttle thread to the right. Does the thread easily slide through the knot? If YES then you have successfully tatted a double knot using the Reverse Riego Method of thread manipulation. Congratulations!