tefilin - How to adjust the knot on tefillin shel rosh? (double daled)

Sometimes the knot on the tefillin shel rosh needs to be adjusted, either due to growth, haircut, or borrowing a pair of tefillin.

How is that knot adjusted?
What piece of the retzuah (strap) should be pulled, in which direction? How would you make it bigger or smaller?

This question is about the double daled knot. For the single daled knot, see this question.

Answer

See this page on Tiferes Stam, which shows a diagram with instructions for both the double- and single-daled knots, as well as information about how the tefillin should fit. The instructions for tightening the double-daled knot are:

Say לשם קדושת תפילין, then

1. Pull the strap marked with the arrow down, making the head loop smaller


2. Close the loop you just made under the knot by pulling up from behind (direction of arrow).


3. Close the second loop, made in step 2, by pulling down on the free-hanging part of the strap.


4. Kesher is complete.