Build the first portal in the Overworld
Note its coordinates
Divide the Overworld X and Z coordinates by 8
Build the Nether portal at the converted X and Z coordinates
Match the Y coordinate as closely as possible
Enter the first portal to create the link
If the portal links incorrectly, destroy the wrong portal and rebuild it at the correct coordinates
Make sure no other portals are too close and interfering
Repeat the same coordinate conversion for any additional portal pairs
Keep portal positions aligned to the same relative locations in both dimensions