Take Route 9 to Route 128/I-95. Head north to exit 28B, Trapelo Road/Lincoln. You will drive approximately 3 miles to Lincoln center, a five road intersection. Sandy Pond Road is directly ahead. The DeCordova Museum is approximately 1/2 of a mile on your right.
You can also take Route 9 to Route 126 North or Route 27 North, which eventually merges with Route 126. Follow Route 126 north towards Concord, take a right on Baker Bridge Road, which is 1 1/2 mile north of Route 117. At the end of the road, take a right on Sandy Pond Road. The Museum will be on your left.
You might find it helpful to print these directions and bring them with you when you travel.