Where, exactly, on the Duamish are you starting from? The answer from ngateguy is accurate, but getting to Fort Dent, if you don't know where it is, might need some explanation. Are you starting up by Boeing Field? Further North or South?
Once you come through Fort Dent Park, you exit onto a street. Get on the South side of the street and find the trail entrance. Taking that trail South using the left fork will take you down past the amusement center and onto the Interurban Trail. Taking it to the Right will take you under the street, coming up on the other side of the intersection. Bear left and you'll go down past Southcenter onto the GRT. Just past the parking lot, you'll again have to turn left to get onto the trail (before the office buildings).
Have they re-opened the trail just past Costco down there, or do you still have to detour through that intersection?