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