Basicallu as Kirkl Said. I find them alot this time of year on the breaks of Umptanum creek or above it on the north side. It tends to be cooler. Durr road then down across the creek and climb the mountaintowards the tower. About half way up, pull off and either glass your butt off into the fingers coming off into the creek or start hiking along the tops on that side. Bare in mind they don't stand out like sore thumbs or like they show in the movies. I don't see them down on the river much this time of year, though I just about hit one coming off from Selah Buttes down to the river less than a week ago.
P.s. That is LOADED with rattlesnakes, and they are moving alot now back towards home and they are in the dog days and not rattling too well so pay attention.