This achievement is gained once you have successfully created a working nether portal and entered into the nether. This must be done is survival. It can be completed on any difficulty.
Flint and steel is created with an iron ingot and flint.Method 1, the Mining Method:
1. Go mining, getting iron, and in then diamonds (see DIAMONDS! achievement guide for more help if needed).
2. Create a diamond pickaxe.
3. Find lava and water. If there is no water naturally nearby to the lava then create a bucket and fill it with water.
4. Apply water to lava (make sure water comes into contact with a lava source block) to create obsidian.
5. Mine the obsidian using your diamond pickaxe. You need 10 obsidian blocks.
6. See below picture for building nether portal frame and place the obsidian just as in the picture.
7. Light the portal with a flint and steel by using the flint and steel on the top side of the portal's base obsidian blocks.
8. Go through the portal for the achievement to unlock.Method 2, the Bucket Method:
1. Create at least two buckets. Fill one with water, and the other with lava.
2. Build a mould for the nether portal so that lava can be contained at each position of an obsidian block.
3. Pour water on the source block to create obsidian. This obsidian should be in the exact place you want it to be for the portal frame.
Note: I suggest you do one block at a time here in order avoid problems.
4. Once the portal frame has been created de-construct the mould and light the portal with a flint and steel by using the flint and steel on the top side of the portal's base obsidian blocks.
5. Go through the portal for the achievement to unlock.Portal frame:
Both frames here are viable, however the one on the right requires four less obsidian blocks, and hence is the better option.