A nether portal is a rectangular obsidian frame lit with flint and steel. The smallest one takes ten obsidian; the corners are optional, which is where the saving comes from.
Building it
- Place four obsidian in a row on the ground.
- Build two columns three blocks high at each end.
- Cap the top with another row so you have a frame five blocks tall and four wide, counting the corners you did not place.
- Light the inside face of the bottom obsidian with flint and steel.
The purple swirl appears and you can walk through. Standing in it for a few seconds moves you between dimensions.
No diamond pickaxe yet?
You can build a portal without mining any obsidian at all. Pour a bucket of lava into a mould dug out of the ground, then pour water on it — the lava turns to obsidian in place, already in the right shape. It is fiddly but it works long before you have diamonds.
Where your portal comes out
This is the part that surprises people. The Nether is compressed eight to one, so your X and Z coordinates are divided by eight when you cross. An Overworld portal at 1000, 64, -320 pairs with a Nether position around 125, 64, -40.
If you do not build the second portal yourself at that exact spot, the game generates one wherever it likes, and two Overworld portals a few hundred blocks apart can easily end up linked to the same Nether portal. Use the nether portal calculator to get the exact coordinates before you go through, then build the matching portal there.
Why bother with the Nether
Because every block you walk there covers eight in the Overworld. A four thousand block journey between bases becomes a five hundred block tunnel. Long-distance transport networks in almost every serious world are built this way — ice roads through the Nether, portals at each end, all aligned with the calculator.
Bring fire resistance if you can, light the tunnel as you dig, and never dig straight into a wall you have not checked. Lava sits behind more of them than you would like.