~10 min
A Nether portal is a rectangular obsidian frame, at minimum 4 blocks wide by 5 tall (a 2-by-3 interior opening), and at maximum 23 by 23. The four corner blocks aren't required for it to activate, even though naturally generated portals always include them. Obsidian itself needs a diamond or netherite pickaxe to actually drop when mined. Once the frame is up, setting the empty interior on fire — with flint and steel, a fire charge, lava next to a flammable block, or even a ghast fireball — fills it with swirling purple portal blocks.
Horizontal distance compresses by a factor of 8 between the two dimensions: moving 1 block in the Nether covers the same ground as moving 8 blocks in the Overworld, while your Y coordinate stays exactly the same in both. Step into a portal and the game takes your X and Z, divides them by 8 (or multiplies by 8 going the other way), and searches near that converted point for an existing portal before creating a brand new one. Two Overworld portals built close enough together can end up converting to nearby enough Nether coordinates that they link to the very same portal, which is the usual cause of "why did I end up somewhere I didn't expect."
Beyond its mobs, the Nether itself is a hazard. It's built almost entirely from netherrack, soul sand and basalt over seas of lava that are far more common than any water — and water doesn't behave normally there anyway, since a bucket of water simply evaporates on contact with most Nether blocks. Beds and boats explode instead of functioning at all. The biomes vary sharply too: Nether wastes and soul sand valleys are relatively open, crimson and warped forests are dense with fungus growths, and basalt deltas are jagged, cramped, and especially lava-heavy.
Around X=50, Z=100 — each coordinate divided by 8, since that's how the game converts your position when finding or creating the linked portal.