Tat the first part of the chain in the normal way, to the point where you want the two shuttle threads to meet. Join the core thread to the base of the first ring with a lock join and leave enough thread to work the 5 stitches needed to complete the chain, back towards the last chain stitch worked. Always leave a little less thread, as this will stretch as you work the backward stitches.
Using the core thread shuttle, pull a loop of thread, from the back, to the front, taking great care not to twist the loop. Thread the shuttle through the loop from the back to the front.
Gently pull the shuttle thread until the loop is much smaller and then push the loop under the chain towards the back.
Pull the loop at the back, gently reducing the thread and you will see the first half of the stitch taking shape.
Draw all the way until the first half of the stitch is tightly into position, taking great care not to twist the new loop that is forming.
Thread the shuttle, from the front to the back, through this new loop. This forms the second half of the stitch, therefore draw up the excess thread until the second half sits snuggly against the first half.
First double stitch completed.