Another in-built turtle command, which is probably much better for resource gathering, is "excavate".
Create the turtle, put it back in the grid with a diamond pickaxe to create the mining variant (other gemmed tools may work too), then place it. Open it like a chest to view its inventory and give it commands. Escape closes it.
Start out with the command "label set turtlename", where turtlename is whatever you want to call it. Naming your turtle ensures that it'll remember certain things if you pickaxe it and later place it somewhere else (like its fuel level, for starters).
Put a stack of coal/charcoal in its inventory and type in the command "refuel all". Alternatively, if you've got EU to spare (it doesn't need all that much, 10k is fine) build a charge station and place the turtle next to that a while.
Put the turtle somewhere you want to mine from and place a chest directly behind it. Give it the command "excavate x", where x is width/length of the hole you want it to dig.
It'll go to bedrock or until it runs out of fuel, whatever happens first. Whenever it fills its inventory it'll return to the chest and dump off its load before going back down to continue digging. It doesn't care about silly things like mobs, lava, or gravity, but you'll have to manually collect and restart it if its chunk happens to unload - keep this in mind before telling it to dig out a 32x32 area or something larger then you've got time to wait for in a sitting. Either that or set up a chunk loader.
You'll probably want to spend some resources on upgrading its storage chest. Wood is easy enough to acquire but it'll fill it very quickly. For larger digs, even a diamond chest will require some watching to prevent overflow.
Again, turtles are slow, but you can have more then one digging at once (gathered around the same chest if you like) and they really do cost very little for what they do. The space they clear for you is almost as valuable as the resources they bring up.
