Instructions

1

Gather several buckets of lava. Farming obsidian requires one bucket of lava for each block of obsidian. You can find large lava lakes deep underground in naturally occurring caves, though it can appear in small quantities above ground. Keep several empty buckets in your inventory in order to gather more lava in a single trip.
2

Fill one bucket with water. Water is much easier to find than lava, as you can find it in small pools and larger lakes across the "Minecraft" world. Unlike lava buckets, obsidian farming will not expend the water, so continuous farming only requires a single bucket.
3

Build a trough to house the lava blocks. The trough should remain one block wide, but you can stretch it up to seven blocks in length. This restriction is based on how far water can flow on an even surface, but you can stretch the trough over longer distances if you build terraces to allow the water to flow continuously downward.
4

Remove the bottom blocks of the trough and replace them with the contents of the lava buckets. Place each block next to the other to form a continuous and flat lava surface on the bottom of the trough.
5

Pour the water bucket out over the trough so that the water flows over the lava. As the water contacts each lava block, it will convert immediately to obsidian. Remove the water into the bucket when finished to expose the new obsidian blocks.
6

Mine the obsidian using a diamond pickaxe or keep it in place for later harvesting. Do not break the obsidian with any tool other than a diamond pickaxe or the block will disappear rather than mine correctly.