The next thing you're going to want to do is make the part that stays on your arm and launches the blade. Start by taking your toilet paper tubes and cutting them in half from top to bottom (see picture). I found it helps if you score it lightly a few times before cutting it.
Set one aside and put the other two together.
Cut/poke a small slit through both rolls, about big enough for a brass fastener. Insert said fasteners.