In Minecraft, one of the most efficient ways to travel across large distances in the Overworld is to travel that distance in the Nether. This is because one block in the Nether is equal to (roughly) eight blocks in the Overworld, making it very important for late-game players to build "Nether hubs" and learn how to link Nether portals.
Nether portal hubs are essential for any late-game player, especially in single player worlds. They are also very helpful to have on multiplayer survival servers or realms that don't have warps or homes. It's a good idea to build your hub in one of these places:
Just below the Nether roof: Get as close to the bedrock roof of the Nether as you can, then tunnel through the Netherrack to where you need to go. This is the safest option before you can get above the bedrock roof.
Above the Nether roof: This is the safest and best place to put your Nether hub, as long as you remember to spawnproof all the blocks you place. There are many tutorials on how to achieve this, mostly involving ender pearls and a small redstone machine; just make sure you're looking at a tutorial for your version!
You should always center your Nether portal hub on your home portal. This means that you'll need to move that portal to wherever you're building your hub - near or above the roof. To do this, follow these steps:
Go to the Nether using the portal. Make sure to bring obsidian, a pickaxe, and a flint and steel!
Break the portal in the Nether. Once it's unlit but before breaking it entirely, write down the coordinates of the two inner blocks.
Tower up above your broken portal into the Nether ceiling until you reach bedrock.
If you're making your portal above the Nether roof, now is when you would get up there.
Clear out an area in the Netherrack, then place your new portal on the same X and Z coordinates as your old one. When you light and go through this portal, it should link up to your original Overworld portal.
How does it work?
Travelling this way works because one block in the Nether is equal to eight in the Overworld. If you were to build a Nether portal at (0, 0) in the Overworld, go through it, and then build a portal in the Nether at (0, 1000), that second portal would put you at (roughly) (0, 8000) in the Overworld.
Building a "linked portal" - a Nether portal that takes you to a specific spot - is actually fairly simple.
First, travel to the Overworld location you want to link (for example, a stronghold).
Build a Nether portal wherever you want in that location, then stand inside it and write down the X and Z coordinates of one of the two inner blocks.
Light the portal, but do not go through it! If you go through the portal at this point, you won't be able to link it.
Travel back to your central Nether portal and enter it.
Now that you're in your Nether hub, take the coordinates you wrote down earlier and divide each of them by eight, then travel to those new coordinates in the Nether. For example, if you built a portal at (800, 800) in the Overworld, you would need to go to (100, 100) in the Nether.
You can also use a Nether portal calculator for this, like this one.
Once you've reached the coordinates, build your portal exactly on them. The Y coordinate doesn't matter, but you should try to match at least one of the two inner blocks exactly.
Now light the portal you just built and go through it! It should take you directly to the portal you built earlier. This will not work if you didn't light the Overworld portal, or went through it prior to building the Nether portal.
My portal put me somewhere else in the Overworld!
Minecraft's Nether portal mechanics can be very finicky, and this is a common experience. If you went through a Nether portal that was supposed to link but didn't, first check the following things.
If your portal generates a new portal in the Overworld:
Was your Nether portal built on the correct coordinates? Check the negatives and make sure you aren't building in the positive X or Z when you should be in the negative, or vice versa.
Did you light the Overworld portal? If you don't activate the Overworld portal before trying to go through the Nether one, it can prevent the link from establishing.
In both of these situations, you need to first break the undesired portal, then correct the mistake (either move the Nether portal or light the Overworld portal) and then try again.
If your portal puts you at a different Overworld portal that you previously built:
The old portal and the one you want to link are too close together. Remove the old one and try again.
When I go back through the Overworld portal, it puts me somewhere else!
In this case, your Nether portal linked properly, but when you go back through the Overworld portal, it puts you at a different Nether portal.
If your portal sends you to a Nether portal that you didn't build:
This occurs because you went through the Overworld portal before the Nether one, so another portal was generated that also links to the same Overworld portal. Usually, this portal will be directly below the one you built.
To fix this, simply remove the undesired portal and try again.
If your portal sends you to your central Nether portal, or another one that you built previously:
The portal you wanted to link and the old one are too close together. If the old portal is properly linked to an Overworld portal, this shouldn't happen, but it still can. There's no easy or quick fix for this, since it takes some trial and error.
To fix it, try deactivating both portals, then relight the one you're trying to link and repeat the steps until it does. Then relight and relink your original portal. If this doesn't work, try it the opposite way.
If everything gets confused, deactivate all the portals and start over.
If it continues to not work, the portals are simply too close together, so you won't be able to link the area you're trying to go to.
As a late-game method for travelling, Nether hubs can seem daunting to create, but in reality they're very simple. Even getting above the Nether roof and breaking a hole in the bedrock can be easy, depending on your game version. Hubs like these are by far the most efficient way to travel the Overworld, regardless of your travel method - whether you use redstone machines, an Elytra, or boats on ice.