If you're really going to do this then read the entire set of instructions before you begin because some things are revealed later that you should be aware of earlier. This is done for reasons of brevity and conciseness. This is already long enough! Maybe that is why this technique seems tricky. Once you know it all it is actually pretty simple and easy.
I'm pretty sure with my instructions anyone can do this, but if you've any questions well, leave a comment, and I'll try to elaborate on anything you are unsure of.
Step 1: Do a Jig
I don't mean any Dancing With the Stars sort of thing. No, to make box joints a fixture jig is used.
Hopefully the graphics here will explain all but if not this is how I make mine.
First I take a piece of wood and screw it to my miter gauge for my table saw. Make sure the wood is flat flush with the table.
Now determine the width of finger joints desired, set a dado blade accordingly. Put blade into the saw, set height etc.
Run the miter gauge with the piece of wood screwed to it over the blade making a notch.
Remove wood from miter gauge and carefully measure the notch.
Step off the measurement of the notch and cut an identical notch into the wood piece.
Make a square pin piece that fits right into your new notch and affix it into there using glue, maybe a nail or something too.
Reattach your completed jig back onto your miter gauge exactly where you took it off. If you need to make a pencil mark to get it right back where it was whatever it takes. It is vitally important that your jig is in the exact right spot on your miter gauge though.
The accuracy which you construct your jig will transfer to the box joints you will make with it so do a good job. I measure my notch with calipers accurate to .001 of an inch. I'm not really too sure if anything else would work. But you're more than welcome to try. I undersize the stuck out part of my pin a little to make it a bit easier to get the box parts on and off it. I just sand the pin a little after it is put together.
The astute observer will notice that the jig is a pattern of one box joint that we will simply repeat over and over in order to cut the successive joinery.