Just run your tap down the hole gently, working it back and forth and when you are done stick one of those little extendable pen sized magnets down the hole and pull out the shavings. Good to go.
Bottom tapping just threads the hole to the bottom of the hole (hence the name). A regular starter tap is tapered so it doesent cut into the base metal all at once, kins like a sheet metal screw. Tapered to fit into the hole and then hog it out to where it needs to be. So, the bottom of the hole where the tap stopped is tapered, and a bolt will not make it passed thes threads.