There is no way to know which way to go for one unfortunately. In my current single player world I had to wander around for well over an hour before I found my way into one, and it's a good 6-7 minute walk just to get there even having a torch marked path leading the way. In the end, it comes down to your luck. Just make sure you don't lose your sense of direction. Not being able to find a fortress is annoying, but not being able to find your portal is just horrible.
Nether fortresses generate in rough, jagged "strips" that run along north-south lines. This means you shouldn't explore for them in a north-south fashion, lest you remain forever stuck between two of these strips, never crossing over one and so never seeing a fortress. Instead, you should explore either east-west or diagonally; this will ensure that you cross over the strips at regular intervals, maximizing your chances of spotting a fortress.
Since learning this trick, it's never taken me more than ten minutes or so to find a fortress, once I start looking for one.
To find direction easily in the nether, place a piece of cobblestone or netherrack, and look at its top face. Both blocks have a distinct "L" shape in the default texture, the arms of which will always point west and north.
What does that mean? I think you mean "follow your z coordinates," by which I think you probably mean "follow the z axis," which won't even help. The z axis is the north-south axis, so (as per my post above) you'd actually want to follow (by which I mean travel parallel to) the x axis, east-west, or else you may never see a fortress if you go north-south instead. Even going diagonally is better than following the z axis.
What I do is build a portal, go through, if there's no fort around I go to the overworld and relocate the portal.
This seems a pretty inefficient way of doing things. Considering you'd have to move your overworld portal 1024+ blocks away from your first one to even come out in a new nether-side location, and even then you'd only be roughly 128 blocks away from your other portal inside the nether.
I just wonder around aimlessly leaving a trail of cobblestone so I don't get lost cause its horrible unable to find a portal. But I'm working on a mod that adds a item that locates the nearest peice of netherbrick.
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Nope.avi. NO! NO SIGNATURE! I AM NOT A SIGNATURE! THIS IS PATRICK!
use AMIDST, its a program where you enter the seed and generates a map of your world (also you can select a level.dat and it will take the seed from there) and you can activate nether fortresses viwew, you can also see jungle temples, desert temples, witch huts, and strongholds, but you cant find dungenons D:
I see a lot of people like cheating. This saddens me. If you follow what IronMagus said you'll find one in no time. I normally do the diagonal trick and find them in no time. I nornally try to search along y = 63 also because that seems like its the average height for most of them and its much easier to see things from that height.
Also to the op, if you've found three already just go to one of them and go north or south for a little while you'll easily find another, I've noticed they are generally only about 300 blocks, give or take 50, away from each other from edge to edge. Keep in mind you might not actually see them because they can generate inside of mountains.
I usually just wander through the nether going "I don't care what I find, just let it be something intersting."
I got my Nether Fortress simply by thinkning that and wandering. I found it in 3 minutes, digging around and listening for piggymen.
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My avatar-Madison Gotha. She knows various weapons, she's a mage/sage, and is somewhat immortal. Madison has the capability of taming and communicating monsters, a genetic trait passed down by her father. 26 physically, 1400-something mentally, and a Lofty Peakian/Zenithian mix. Doesn't really like being called an elf and hates being called human..
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