Page 104 - Patrick Moreau, Jean-Benoit Heron Marine Knots How to Tie 40 Essential Knots

1 - Classic beginning with an initial large half hitch, working end passing over.
The working end is then led under twice.
The large bight below is the final eye.

2 - Lead the large bight under the small one, which is located twice under, and finally pass it over.
Adjust it so that the construction is quite compact.
You can also make an additional twist with the bight of the final eye.
This gives you the double harness knot.