To get the Into The Nether achievement easily, first of all you have to go into the tutorial mode. (This Tutorial Mode is in the latest Update) You will spawn outside a Huge house, enter the house. One of the rooms will have an enchanting table and a chest, open the chest and you will find a full set of Diamond tools. Next, you have to go to the minecart beside the house. when it stops, get off it turn around (so you are facing the house) and take a few steps forward. then dig into the ground, and once you are about 4 blocks deep, dig forward and you will find a cave. In the cave you will be able to find Iron. Mine the Iron. You will also find some gravel, mine th gravel until you get flint. Once you get it return to the house and craft flint and steel at the Crafting table.

Go upstairs and enter the bedroom, you will find a lever, pull the lever and you should hear a noise. Go back into the hallway and you will see a passage in the wall. Enter and you will find a nether portal, Get your diamond pickaxe and break ONE block of the nether portal and place it back. Then ignite the nether portal with your flint and steel, and you should get the achievement Into The Nether!