Gather 10 obsidian blocks
Get a flint and steel or another fire source
Build a rectangular frame 4 blocks tall and 5 blocks wide with the inside empty
Place 2 obsidian blocks on the ground for the bottom
Stack 3 obsidian blocks on each side
Place 2 obsidian blocks across the top
Light the inside bottom of the frame with flint and steel
Enter the purple portal to travel to the Nether