Actually, it is quite simple for the dissaffected counties to split from the home state, that's just a vote of the county commissioners. Problem is, once they split they're back to being just a territory of the US and would have to appeal to Congress for admission as a new state. The other potential problem is "checkerboarding", say if most of the counties of Eastern WA voted to succeed, but Spokane County voted to stay in Washington, what to do then?

As for the new state, I would pitch the idea not only to the counties of Northeastern Oregon and Northern Idaho, but also the counties of Southwestern Washington. Sometimes voters in Vancouver and Longview feel just as disenfranchised by Puget Sound as the eastsiders. It'd be nice for this new state to have it's own deepwater port accessable by both BNSF and UP as well as the barge lines and interstate truckers!

And then there's the problem of which city/site to choose for the new state capital. Depending on which counties within and without the state of Washington opted for the new entity, I'd opt for Ritzville, Walla Walla, or Prosser - whichever one is most centralized to the new state boundaries. The larger cities - Spokane, Tri-Cities, Vancouver - would be out in order to keep the balance of power from being monopolized.