I've had my best luck catching coho using small flash flies, generally a size 8. If you're not catching them using one color, try something else. I generally do best using either green/silver, blue/silver, and straight silver. I also like the flies that have a bead head as this is imperative to getting the fly at the correct depth. Another good series of flies are the Rolled Mudler minows. I've had my best luck using farely small, sparse flies.
As for leaders, I use a Rio line system that has an "intermediate" head. This head is clear and about 15 feet long. I attach a 9 foot salmon/steelhead leader to this. If you just have a floating line, use a long leader and bring along some small spit shot.
You'll want to mix up the retrieve, varying the amount the fly "flashes" from 4 to 12 inch strips. Cast toward rolling fish.
Coho are odd, one day they'll take anything and are super aggressive. If you're not successful casting to rolling fish it could be that the fish could be in a state where they'll refuse everything. Some days they seem to hammer everything in site and then litterally the next day, they won't hit anything.
My experience has been that Coho are fairly spooky and so I would suggest that you stay at least 50-60 feet away from rolling fish. If you cast well, you'll probably want to stay 80-90 feet away and cast to the limit of your casting ability. One things for certain, you'll want to make sure that your casts don't slap the water or you can spook the whole pod.
I actually haven't fished the Green but have fished numerous rivers in Oregon, Washington, Brittish Columbia and Alaska. I've also chased coho in the salt water in Puget Sound, the Straight of Juan de Fuca and off of the Oregon Coast. In my opininion, coho are the ultimate game fish. I learned the techniques and flies to use from a guide that lives in B.C. and they've been successful all over the northwest. Speaking of which, I'm heading to B.C. next week for 7 days of coho fishing up on a tributary of the might Fraser River!!! Can't wait.
You might have somewhat of a hard time finding small flash flies and/or rolled mudler minnow patterns. There are a some fly shops out of Surrey, (look at myflyshop.com) that have these flies. I haven't been able to find them other than there so I end up tying my own. They're very easily tied, even for the beginner.