The actual starting point is at Donovan Ave and 10th Street in Fairhaven, although there are several locations where you can start. You can identify the trail by the short brown posts with a yellow strip around the top.
Follow the trail beside the creek, bypassing Fairhaven Park and crossing multiple low-traffic roads. (For a starting point with more parking and toilets, you might want to start at Fairhaven Park and pick up the trail there.) When you're nearly at the intersection with Old Samish Road, you'll take a y-fork to the right. (The trail going straight ahead is a dead end.) Follow some switchbacks down to the road.
You would normally cross Old Samish Road onto the Arroyo Park trail and follow it upstream to a bridge, but in January 2021 a tree fell on the bridge; it's now closed and blocked.
For now, you need to head along Old Samish Road to Chuckanut Drive, turn left, and regain the Interurban Trail by following signs at the North Chuckanut Mountain Trailhead. It's a somewhat steep but well-maintained uphill trail. You'll turn right to get back on the Interurban Trail.
After you cross California Street, the trail flattens. As you continue, you'll have great views of the San Juan Islands to your right. Pass the Cleator Road
parking lot and head down and up the "dip" to yet another flat section. Pass the Fragrance Lake Trail
on your left, and soon the trail will end next to the Lost Lake parking lot.