Gather at least 10 obsidian blocks
Get a flint and steel or another fire source
Build a rectangular frame 4 blocks tall and 5 blocks wide using obsidian
Leave the inside of the frame empty
Place the obsidian so the frame is 2 blocks wide and 3 blocks tall inside
Ignite the bottom inside of the frame with flint and steel
Step into the purple portal to enter the Nether