Gather at least 10 obsidian blocks
Get flint and steel or another fire source
Build a rectangular frame 4 blocks tall and 5 blocks wide with the inside empty
Place obsidian in a 4×5 outer frame shape
Make sure the inner opening is 2 blocks wide and 3 blocks tall
Light the inside bottom of the frame with flint and steel
Enter the purple portal to travel to the Nether