1) Because the joint slots need to be cut in the end of pairs  of boards, slotted to fit together, it's necessary to clamp board sets to a guide that will run along the router table fence.  

 

Here I'm making a box with 6" (1/2" in 1/12 scale)  I'm using a 1/4" router bit. This requires two slots, spaced 1/4" apart. If you use an odd number of box fingers, you'll need to index the two boards.

 

 

 

 

 

2) Set the router bit such that it is just a bit more than the wall thickness above the router table. Plan for this additional length as it needs to be accounted for when you layout the box sides. 

 

This will generate fingers and slots that are a bit too long and these can be sanded smooth after the joint is assembled.  With the guide held against the fence, start the router and push the parts into the router.  Once the router passes all the way through the parts and into the guide, you can back up, adjust the fence for the next set of slots and repeat the process.

 

 

 

 

 

 

 

3) The joint can be glued together.  I use a magnetic board, magnets and a small square to ensure that I get a nice, 90-degree angle.  Note how the extra-long fingers are sticking out at this point.

 

 

 

 

 

 

 

 

4) Once the glue has dried, you can sand the exterior walls, removing the excess stock that's part of the finger joints.  Here I'm just using a sanding block to do the job.

 

 

 

 

 

 

 

5) Voila!  Almost instant box joints.