Make sure you have both stones, one in your inventory and one on the ground at another location. You can place one a few hundred yards away to test the stones
The first time you drop the stone, press the Skill Browser icon on the UI. Select the Drop and Use Icon. See attached image.
This should "activate" the stone and teleport you to the other stone. Once at the other stone simply click the stone to return to your original location. Put the first stone back into your inventory by opening the inventory, click and hold on the stone, drag it to your inventory and release it.
Some like to leave it on the ground after returning since clicking it will return you to the other location. (this works well if you are teleporting in and out of a battle) I find it too easy to forget the stone, leaving it somewhere I can't remember. I always put the stone back in my inventory.
If you do loose a stone, go back to your other stone, put it in your inventory and reactivate it (Drop and Use Icon) to return to the lost stone.
BTW, once in the Wastelands the stones will be call Ancient Pyramids but they are the same as the teleport stones.