The store you buy it from should have the brochure they're supposed to give you that details all of the specs for installation - see if you can get it from them.
The general plan is, you use metal roofing screws (they have a washer and gasket), and you really ought to buy the wavy profile piece that fills the gap between the palruf panel and your roof or nailer -- that way you can screw thru the RIDGE (not valley) thru the profiled filler piece and into your roof wood. Screwing on the ridges not valleys makes the roof much much more leakproof - even with the gasketed washers if you screw in the valleys you WILL get rot gradually forming around each screw, and may even get leaks into the coop.
It would be best to be screwing into a decent amount of material. Optimally you would have 1" or more of actual roof wood, meaning nailers under your plywood. Failing that, if you are not in a very windy place you could prolly get along fairly well by securely screwing the filler strips to the plywood (put some screws in from the outside, starting them into the filler strip; put other screws out from the inside, starting them into the inside of the plywood roofing) and then use roofing screws that are sized to go all the way through the filler strip and plywood but just not *quite* come out the other side. (you don't want exposed pointy things studding the ceiling of the coop, plus screws don't hold nearly as well if the point goes all the way thru and comes out the other side of the material)
That palruf stuff is really soft and flexible. I do not think I'd overhang it even 3" - you don't want it so bendy that gusts of wind could curl it up a little bit where a *strong* wind could catch it and rip it off/apart. Personally I'd give it maybe a 1" overhang max.
Good luck and have fun,
Pat