AUTHOR'S NOTE: THIS ROUTE SHOULD NOT BE CONFUSED WITH THE APPALACHIAN TRAIL APPROACH FROM THE SOUTH THAT BEGINS ON ROUTE 23 NEAR THE ENTRANCE TO HIGH POINT STATE PARK.
From NY: Take I-84 Exit 2 and proceed south on County Route 55 (AKA Mountain Road). This road crosses into New Jersey approximately 3.3 miles from the Interstate and becomes Route 519 but keeps the Mountain Road moniker. The Appalachian Trail crosses the road approximately 2/10 of a mile into New Jersey. Park on the shoulder of the road.
From NJ: The State Route 23/County Route 519 split is located approximately 1 mile southeast of the entrance to High Point State Park. From this point, follow Mountain Road (County Route 519) north for 2.3 miles to the Appalachian Trail crossing. Park on the shoulder of the road.
The Appalachian Trail crossing sits at an elevation of about 1100 feet and ascends very gently through this birch tree-lined section of trail. Begin your trip by leaving the west side of the road onto the "southbound" portion of the A.T. The only way to describe this hike is as pleasant. The average hiker should require no more than one to one and a half hours to cover the approximately two miles from the trailhead to the 1803-foot summit of High Point. The only caveat here is that the hiker is required to leave the A.T. just shy of the summit of High Point and follow the Monument Trail the final quarter of a mile to the summit. To reach the Monument Trail, turn right (north) at a trail junction where the Appalachian Trail turns 90 degrees to the left (south). This point is just shy of Lake Marcia. Hike uphill less than 200 feet to the top of New Jersey.
A detailed map of the area can be obtained from the New York/New Jersey Trail Conference. The appropriate map for this hike is the North Kittatinny Trails map (number 18). A four-set Kittatinny Trails map is available for $12.95 and can be obtained here.
Added by RobA regarding Monument Trail:
The current blazes for the Monument Trail in red, but it used to be blue. When it was blue, it took a different route to the top than it does today. Not all of the blue blazes have been covered up. Make sure you follow the red blazes, the blue ones stop in the middle of the woods 100 yards after the A.T.