If you're coming by Metro, Smithsonian is the closest stop. Make your way on foot to where 17th St. SW and Independence Avenue SW intersect by the World War 2 Memorial. From the World War 2 Memorial parking lot cross over Independence Avenue. The first stop is going to be the grassy area located to the right of the bridge crossing the Tidal Basin.
There is street parking located on Ohio Drive SW. If you're coming from that direction. Walk down Independence past the Martin Luther King Jr. Memorial. The first stop will be on your right.
If the trees are in bloom, going by foot will be your best bet. Driving in that part of town can be a nightmare at that time of year.