Gather obsidian blocks
Make a rectangular frame 4 blocks wide and 5 blocks tall
Leave the inside hollow
Place flint and steel in your hotbar
Right-click the bottom inside of the frame with flint and steel
Enter the activated portal
For a Nether portal, use at least 10 obsidian blocks
For an End portal, find a stronghold
Locate the End portal frame room
Place Eyes of Ender into each portal frame block
Step into the activated End portal