This is the video I found the most helpful for the button knot. Many different ways to splice the shackle itself, I settled on a single pass loop (just first part of locked brummel) and a button knot with tails burried.
The knot was the most challenging part for me to learn.
Made a few this weekend from 5/16". All started with the same cut length of rope. Small differences in noose eye size, how close the button knot is to the twine constrictor knot when done, knot setting, and length of tails/burry result in some OAL variation.
I think the main benefit to the button knot is that the tails come out along the legs of the shackle so they can be burried. End up with 4 legs of rope for the noose loop to lock onto (more shear strength and larger bend radius). Once you get the hang of it, its not bad to tie. Takes more time to dress and set it well.
The final form before you tuck the tails down through the crown and along the legs of the shackle. The next step is pretty clear in the video.
Tried to get some pics of final part. May be better to watch video.
Same pic as above, just snugged up a bit and held in my hand looking down the top. Thumb and index finger are on front and back of shackle legs (inside the knot).
Open up crown. You can see my fingers (black spots) are where the tails need to go.
Now insert tails. Bottom right to top left. Top left to bottom right. Imori ends up where thumb was. Blue Moon ends up where index finger was.
Not a very clear pic but this is the complete knot.
From the bottom side.
From top.