How do you make a Portal to the Nether?

Answer 1

make a 4 by 5 square made of obsidean. then put flint and steel on the bottom. if my measurements are wrong, look on the minecraft wiki.
If you had a 4 by 5 grid, 4 being horizontal, while using the least amount of obsidian: Top left: Nothing. The one Right of that: Obsidian. Next: Obsidian. Next: Nothing. Do this for the top and bottom rows. The ones in the middle, do obsidian, nothing, nothing, obsidian. Then light the empty space within the structure with a flint and steel and stand in the portal for a while befor you drift off to the nether
Make a vertical 5 by 4 rectangle out of obsidian with a 2 by 3 hole in the middle and lightthe top of one of the bottom obsidian blocks on fire.

Answer 2

You can't. Even if you build the same kind of Nether Portal frame in The End, that you'd use in the overworld, it cannot be activated. You can only activate them in the overworld. So there is no way to build a functioning Nether World portal in The End.