Make sure the drill bit is at least within 1/8" of the size of your chisel. 1/16" is ideal.
Shoot for a 45 degree angle all the way through the block.
When you're getting close, make sure the next hit won't send your chisel into a hard surface.
Adjust the cutting edge by either tapping lightly on the back of the chisel or the corner of the wood near the chisel.
When using, keep even pressure on the block of wood against your reference face