From the trail head at Glacier Point Parking lot take the Sentinal Dome trail approximately 1.0 mile until you come to the Halfdome view. It's a large opening on the forest trail you can't miss.
If you go in May you will probably need either snowshoes or instep crampons. I carried both but only used the crampons.
View of Halfdome and the Yosemite Valley from the Sentinel Dome trail in Yosemite National Park from the Glacier Point Trailhead