How do I build down?
Place water and lava on adjacent sides of the block.
Top view:
D L
W
When they flow it creates a column of cobble you can dig through.
Side view:
W D L
↓ ↓ C ↓ ↓
↓ ↓ C ↓ ↓
↓ ↓ C ↓ ↓
↓ ↓ C ↓ ↓
D = Dirt C = Cobblestone W = Water L = Lava ↓ = Flowing water/lava
Use water scaffolding.
Check out this video at about 10:00 to see water scaffolding in use.
The lava/water method seems easiest, but if you don't have lava, you could also use a piston to push a bunch of blocks down, one at a time. Then mine safely down at that same spot. Make sure not to mine too far