Inspired by Nether Portal Calculator v1.0 by D3Phoenix http://ilurker.rooms.cwal.net/portal.html
Choose a location for a portal in the OVERWORLD and, without lighting it, build the frame. Walk into your portal
frame as if you were going to use it, and run the command /tnp. You will be messaged the coordinates for placing your
NETHER-side portal. Light your portal and go to the NETHER. Go to the calculated coordinates in the NETHER using F3.
Destroy and replace the block below your feet at these coordinates with obsidian. Turn your character until the F3
Facing (F) number from earlier matches up. This is the direction you will be facing when exiting the portal. Place a
second obsidian block in the floor either to your left or right (it doesn't matter). These two obsidian will form the
base of your portal. Build a walkway from the portal base at least a few blocks in the matching "F" number direction
to ensure a smooth transition through the portal. (This ensures that you won't be staring at a wall after going
through a portal.) Complete the NETHER portal frame and light it. Disable or destroy the NETHER portal that the game
spawned for you when you first entered the NETHER. Exit the NETHER through the new portal you just built. If you did
everything right, you should have a pair of perfectly linked NETHER portals. You can repeat this process as many
times as you like.