Not familar with the Methow. But I'm guessing that coho are coho. On VI rivers, early morning or cloudy days (eg low light) are usually best. They are a funny fish. I would fish with steelhead tackle, either scandi/shagit with various sink tips and 4-9 foot leaders. Good holding water for coho is quite different from good steelhead water (assuming good water temps for the fish). Best water is usually pools, backwaters (even big back eddies). Slow runs are great, and you may get some fish near the head end, but lots will sit where the water is slower than what might be regarded as a good speed for steelhead. For flies, clousers in different colours, your favourite small streamers, and a sleeper; gold-bobied muddlers. Try colours that work on that river, but don't get stuck on one fly if you know you are covering fish. Keep changing! Coho seem to like flies on the drop so always worth trying a strip with a good pause type of retrieve with the clousers (or something else with some weight at the head). Also worth noting that coho, being a fish that tends to spawn (and rear) in small tributary streams love overhead cover. That could be depth, logs, stumps, shade, etc. This should get you off to a good start. Other thing to look for, especially in "interior" streams are areas around springs that cool water down to a coho's comfort zone. Comfortable temperatures are very important. Tight Lines!
Sorry: Retrieve will vary but usually you are looking for agressive ones first so try a strip or strip/pause. If fish are rolling then start switching flies and changing retrieves. Usually, retrieve speed will be dictated by the water you are fishing and of course, the fish. May be highly variable from day to day!