Gather obsidian blocks
Craft or obtain a flint and steel
Place obsidian in a rectangle frame, 4 blocks wide and 5 blocks tall for the Nether portal
Leave the inside of the frame empty
Use flint and steel on the inside of the frame
Step into the purple portal to activate it
Build the frame with at least 10 obsidian blocks for a minimal Nether portal
For a larger frame, keep the same rectangular shape and size limits
Ensure the corners can be left out if needed for a minimal frame
Use the portal to travel between the Overworld and the Nether