From Vancouver:

Go south on HWY 99 through the Peace Arch border crossing into Washington State. Take exit 276 for WA-548 South and Blaine City Center.  Turn right on Peace Portal Drive and proceed through Blaine. Turn right at the four-way-stop onto Bell Road. Turn right onto Drayton Road, then left on Harborview Road, then right on Lincoln Road. Continue up the hill to Semiahmoo Parkway and left into Horizon.

From Seattle:

Go north on I-5 and take exit 270 for Lynden / Birch Bay. Turn left on Birch Bay Lynden Road and proceed through Birch Bay. Turn right on Harborview Road, then left on Lincoln Road. Continue up the hill to Semiahmoo Parkway and left into Horizon.