Minecraft: Edition

In order to construct a Nether Portal you'll need Obsidian and you need 14 Blocks of it. You can create Obsidian by releasing Water into Lava or you can sometimes find it underground in a similar scenario. Once you have it you'll want to construct your Portal 4x5 so 4 along 5 up.
Then get flint which is made by destroying Gravel and combine that with an Iron Ingot and you have the tool to light up the Portal so walk up and use the flint and the Nether Portal will light up allowing you to cross into the Nether.

This is a video I made for my channel. It shows you what to do as well as the On A Rail Achievement.

Like many other achievements so far, if you have an existing 360 save with [required item], in this case a Nether portal, you can cheap out and get the achievement easily.

You'll need your existing portal, a diamond pickaxe, and flint and steel. Use your pickaxe to take out one of the obsidian blocks bordering the purple field. When it breaks, pick it up and replace in the same spot. Use the flint & steel on the open portal "window" and the purple haze returns, your achievement pops, and you feel slightly more guilty than when you built the thing with the 'bucket trick' on the 360 originally...

To create a nether portal you need 10 obsidian blocks. This can be quite a challenge - you need a diamond pickaxe to mine obsidian and obsidian is rare. Obsidian is made from lava and water, so look where you see the two things together to try to find it.

Once you have them, the blocks must be placed 5 blocks high and 4 blocks wide to create a frame. Note that the corners of this frame can be left out. Once this is done, the bottom of the portal must be lit on fire with flint and steel (a fire tool, crafted with 1 flint and 1 iron ingot), then the achievement is yours.

Refer to this video if you're having trouble:

Source:
https://www.xboxachievements.com/game/minecraft/achievement/...
All credit goes to the original writer.