How do you build a nether portal in Minecraft?

Answer 1

you must build it like

R O O R

O E E O

O E E O

O E E O

R O O R

where R is random block, E is empty place and O is obsidian

then you burn fire inside this and you must go in this purple portal

PS: you must use blocks you can not do it on crafting table

Answer 2

You need 12 blocks of obsidian, which you get from mixing flowing water with a lava source block and mining it with a diamond pickaxe. Then you make a 4Lx5Hx1W hallow box and light it with a flint and steel.