Open the Nether
Collect Eyes of Ender
Throw an Eye of Ender in the Overworld to find a stronghold
Enter the stronghold and locate the End Portal room
If you mean a Nether Fortress, travel in the Nether
Move in long straight lines to cover more ground
Search near the lava ocean and open areas
Look for dark red Nether brick structures
Follow bridges, towers, and corridors made of Nether bricks
Increase render distance if possible
Use coordinates to track where you have searched
Keep moving in one direction until you find one