A nether portal is built as a vertical, rectangular frame of obsidian (4×5 minimum, 23×23 maximum). The four corners of the frame are not required, but portals created by the game always include them, resulting in 4 free/extra obsidian. The obsidian can be placed in any manner, e.g. by placing mined obsidian or by casting it in place using lava and water. Adjacent portals can share obsidian blocks. A nether portal cannot be built horizontally like an End Portal.
Once a frame is constructed, it is activated by fire placed inside the frame. This creates portal blocks inside the frame, resembling a vortex. The fire can be placed in any manner, including use of flint and steel or a fire charge, the impact of a ghast or blaze fireball, or natural spread of fire to flammable material adjacent to the portal. Nether portals cannot be activated in The End and customized dimensions.
When a portal is used, if no corresponding portal within range exists in the other dimension, one is created there. There is an exception to the above rule. There is an infinitesimal chance that a portal created in the Overworld does not generate a portal in the Nether, which would leave the player trapped until death or construction of a new portal.
September 7, 2020