Zax is right about the leather smelling terrible. :-)
If you use tape over the mirrors and you burn through inadvertently to the mirror surface then you will get a whole heap of sticky crud on the surface that is hard to clean off. Much better using some thick (1-2mm) card and using the lowest power you can to just get a small light spot on the surface to find the right place for the beam.
If your lens is about 50mm focal length then it is relatively easy to get the right focus. Place a piece of thick card under the lens at an angle so it is about 2-3mm higher on one side with the length about 50mm long. Set the focus to about the average height of the card in the middle and engrave/cut a single line along the length of it at minimum power. You will see the char line start thick then go thin then go thick again. The thinnest part of the line is the focal length of the lens and is easily measured from a certain point of your choice on the lens assembly. make a mark or make a gauge to get the height, it's your choice. I just use a 6" rule and measure to a scribed line on my lens assembly that is 2" from the surface of the part to be cut/engraved.
I am not completely useless.......I can always serve as a BAD example.