If you connect up to the Northern Tier, you will come across WA on Highway 20. If you want to go to Seattle instead of Anacotes where the NT ends, here are some directions that keep you off most of the major roads. After crossing the Cascades, following the NT will get you on the Rockport-Cascades road. At Highway 530, instead of turning right to Rockport, turn left to Darrington. Follow hy 530 to Arlington. At Arlington, turn left andget on the Centennial Trail. The trail through Arlington is mostly a wide sidewalk on the east side of the street, whose name changes several times as you head South (but is mostly 67th Ave NE) At 172nd, the trail ends for a bit, and you have to ride on a road with no shoulders for about 20 blocks. Just short of 152nd, there is a trailhead for the trail again on the left; get on the trail and head south to Snohomish. In Snohomish, the trail ends, and you till be paralleling Maple St. Follow Maple Ave to 1st St and make a right turn. Ride through downtown Snohomish (Fred's Alehouse has lots of good brews) and head to Ave D. Turn left on Ave D and cross the river--this will put you on Airport Road. Follow this aeound the bend till you come to a major intersection. Bear left onto Springhetti Rd. Follow this up the hill, and around the bend until you reach Broadway. Turn left on Broadway. You will ride for several miles on Broadway till you come to the town of Maltby, Just past the community park, Take a left onto Yew Way and cross under the freeway. Take a right on Bostian Rd and parallel the freeway. Bostian becomes 224th and then bends to the left and becomes 75th Ave SE, which then becomes 156th Ave NE as you cross the county line. Turn right onto Woodinville-Duvall rd and go down the hill to Woodinville (note that Woodinville-Duvall Rd bears left shortly after you get on it) W-D road becomes NE 175th St in Woodinville, and continue on it. Just W of 131st, there is an entrance to the Samammish River trail on your left. Go on the trail and turn right. The Samammish River Trail becomes the Burke Gilman Trail, and runs on into the north end of Seattle, past the Univ of Washington, and eventually out into the part of town called Ballard, and Puget Sound. MOst Seattle buses have bike racks, and there is light rail (sound Transit) from downtown to Sea-Tac airport.
If you have any questions, I'll be happy to try to answer them. I live a few miles from downtown Snohomish and am alway willing to put up touring cyclists