The double haul is a casting technique that can be challenging for many fly fisherman. Simon Gawesworth for Rio Products offers some excellent tips.
The double haul is a “pat your head while rubbing your stomach” kinda of cast but if you follow Simon’s suggestions you should be able to get the hang of it.
I agree with the set up recommends to learn the double haul, but you don’t have to wait until you have it. You can learn with a regular weight forward line. Simon’s suggestion will just make it easier.
If you want a casting lesson or Rio lines you can find both at Mossy Creek Fly Fishing.