Minecraft wiki entry for Building_a_metropolis, associated text: How to start a branch mine:
1. Dig a 3×3 staircase (Not going straight down).
2. Figure out what level you want to miners to dig at.
3. Dig a 3×3 tunnel connected to the end of your staircase, it can be off any side(s).
4. Dig 1×2 tunnels in your 3×3 tunnel for your miners to use (Optional: Pre-mine some tunnels).
5. Continue your 3×3 tunnel for as long as you wish and add as many 1×2 tunnels as you wish.