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.
Step 2: Cut Sides
This sounds simpler than it actually turns out to be. This is where some experience will eventually pay off as you perform this task more. You see this joint chews into your wood stock somewhat. In that I mean with a simple butt joint in wood your side is your side, but this joinery uses up some of that stock material in it's nesting nature. What is more is in order to really make this joint well you should overhang the fingers a little. Then once the box is fully done sand the fingers that stick out flush to the sides.
What I am driving at here is your completed box will have sides smaller than your cut sides are. How much smaller depends on the thickness of the wood, and the amount of overhang you allow for with your joint fingers. Initially this shouldn't be all that critical I mean just be happy you're making boxes, but down the road when you set yourself to some more demanding tasks it is something you'll have to work on.
For now focus on making two pairs of equal length sides for your boxes. Oh yes, your sides have to be square cut as well.
Step 3: Perform Test Cuts
Every time I do this I always do test cuts on the stock I plan on making boxes out of. The kind of wood it is, the various adjustments, dado width, and blade height, all have to be dialed in to work with each other. Joints should fit together without being too loose, or too tight.
But if we're going to be doing test cuts I guess I have to tell you how this whole cutting box joints thing works now don't I? This is the abbreviated version, the full version will come later when we're making our actual boxes.
Take your two scrap pieces and set them standing up on your bench like they are the corner of a box.
Put an X or some other sort of a mark on the top edge of each piece where they come together.
Take one and butt it up against the side of the pin on the jig. Run a notch into it. (red in the graphics)
OK now this is the tricky part so pay attention:
Take that first piece (red) you just cut and put the notch in it, flip it over and put it's notch onto the pin. The one tooth should fill the gap between the pin and the notch on the jig. Now take your other piece (blue) and butt it to the edge of the first piece mark to mark. You should notice that if you pass the work over the saw blade your second piece (blue) is going to get it's corner notched out. If that is how it looks to you go ahead and do it, make the cut.
Now you're going to gang cut the two pieces together. Take the first piece flip it back around and put it's notch over the pin of the jig. Put the cut notch of the second piece onto the pin. Make a new notch through both pieces, then put that new notch onto the pin, notch, move, notch, move until you've gone the length of the pieces.
Now line up your marks again and put the pieces together. Don't tell me you marked the second piece where the saw cut the mark off. ha ha I do it all the time! We'll learn to back that mark off someday.
This is harder to explain in text than in pictures, so try to follow along with the amazing graphics I drew. Because it really isn't as hard as it sounds.
Step 4: Lay Out Sides
Now hopefully you can still count to 10 without having to take your socks off, and you've dialed in your cutting setup too. If I am making a box out of one piece of wood I like to have my sides all in order so I line them all up so the grain wraps around the box. It is an extra little detail I just like to do it isn't important though.
What is important is that you lay out your sides so they do wrap up into a box.
Step 5: Box of Cards
Some arrange their pieces in a flat cross like pattern I like to prop mine up. Propping them up helps me keep it all straight while I number all of the corners. I number my corners 1 to 4 and put the number onto each piece. Doing this will be important later on.
My numbers are a little hard to see in the image but they're all there.
Step 6: Cut Notches
The sides in the graphic that are the same color have to match. So you're either cutting them all first, or second. Remember butt number up to number in this step while notching. It is just like butting up mark to mark while test cutting.
Step 7: Gang Up
If you got this far you are virtually done. All you need to do at this point is set your notched sides onto the pin and have at it! If your initial notches you made are right then this has to come out right as well. So just cutting down the line twice and you'll have all of your box joints done for a box.
I would just like to add that I clamp my sides together and this gets me more precise cuts. Saves me some effort in that I don't have to grip all the pieces together too. It'll work if you don't clamp all your sides together while you gang cut them, just not as good and it'll be tougher on you to do too.
Step 8: Stacked
As I cut my boxes I like to keep them dry fitted together so I keep all the sets together. I'll leave it to you how you'll handle putting bottoms onto your boxes, tops, etc. I usually just cut a piece to fit and glue it in there myself.
One more thing, I've cut about a billion of these by now and over time I've concluded that you only really need to make two sizes 1/4" for thin walled boxes, and 1/2" for thicker walled ones. I've seen fancy adjustable jigs for making box joints but making one of those is kind of silly to me. Just adds to potential errors and general confusion as far as I'm concerned.
But that is just my opinion you're free to do as you choose.