It's hard to get decent directions to the beach, so I'll tell you what I know. On the Big Island (Hawaii or Kona), you want to take Highway 11 (the main highway) south to Ka Lae. There will be a sign pointing you further south to South Point, which is the southernmost point of the U.S. That road (~11 miles) takes you across windswept pastures, with a wind farm on your right. Eventually you will come to a parking lot, though most vehicles can probably make it a little further to the boat ramp. If you are facing the boat ramp, you will see a 'Government Property' sign on your left, with a trail. That is the path to Green Sand Beach. It is 6 miles from that sign to the beach.
The walk across the old lava bed is pretty easy, but it's almost always sunny, windy, and hot. I'd say it's probably a good 3 hour hike to the beach. I'd recommend a backpack with water. Be sure to wear good shoes. It's not smooth enough for strollers or wheelchairs.
If you can't make it all the way to Green Sand Beach, there is an area that I'll call 'Green-Enough Sand Beach' about 1-1/2 hours walking from the boat ramp, which has the green sand around tide pools.