I use
this for countersinking.
For slots, I drill a hole on both ends of the slot. I then use
this type of bit on a dremel tool. Then, just cut away.
For cutting, I use a hacksaw and/or Dremel tool with cutting wheels (wear goggles!). If thinner than ~2mm, I use a nibbling tool to make nice square cuts and files/Dremel sanding wheel to clean up the edges.