First off, this achievement all starts with a wooden pickaxe.
Upgrade it to stone.
Upgrade it to iron.
Upgrade it to gold. (If you want).
Upgrade it to diamond.
Find and mine obsidian.
You'll need about 16 obsidian blocks.
You will also need Flint & Steel.
Flint & Steel is crafted by iron and flint.
Once you have your 16 blocks and flint and steel, build the 16 blocks like so in the picture.
Once it's all together, light it (so touch inside of it) using the flint and steel.
If you can't figure it out, even though you looked at the picture perfectly, probably best to check a video guide.