The double haul will come with time and practice
Just a matter of timing, speed and feel. Took about a year to pick it up.
Now I would have a hard to to explain how to do it on paper
Like a good golf or baseball swing hard to explain.
I double haul with my 6 1/2 foot fiber touch 2 wt with a slight double taper line to my looomis in 12 wt with a shooting head for tarpon. Completely different each rod but I know the feeling and how to execute.