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>How do I convince them
It's supposed to be a fun activity, so I can't imagine your friends should take much convincing. Unless they're a bunch of cunts.
>how difficult is it?
This depends entirely on how large and complex you would like your fort to be. I did this when I was in my teens and it took maybe a day and a half to build a fairly large large fort just using paracord and logs (some precut we scavenged, some standing deadwood that we took down with hand saws) with 2 friends. Was pretty fun building it and we cooked out there, smoked and drank, and even did a couple of overnights. Came back a couple of years later and someone had destroyed it. Such is life.
Just get a few crates of beer, roll a couple of blunts and head out into the woods, it doesn't have to be complicated.