The input window for the extension leaves you plenty of options for the joint design. In addition to the size of the box (inside or outside measurements can be selected), you need to enter the thickness of the material, the kerf size (how wide the cut will be), and how much clearance...
All parts are now ready to be cut. Like I said before the laser I use will only cut the lines that are in red. To make all lines red use Edit-->Select all and then Object-->Fill and stroke…, this will give you a popup window. Select the Stroke paint tab and pull the red sl...