There are all kinds of commercially available jigs and tools to assist in making box joints (or dovetails) but I made my own jig quickly and easily with the router table at TechShop and some scrap wood.
Router table and a 1/4" straight bit
Compound miter saw or table saw
3-4" wide board
I made it at TechShop!
Step 1: Prepare the Sides
Cut four sides. Label each piece lightly in pencil: front, back, left and right. It helps to put the label at the top of each piece to aid in orientation later.
If it helps, lay them out in the way they will be assembled.
Step 2: Prepare the Jig
Take a piece of flat scrap wood and clamp it to your miter gauge. In my case, I needed an additional piece of scrap to provide a good clamping surface on the rear of the miter gauge. The front piece is flush with the surface of the router table, the second, clamping piece, clears the router bit.
Turn on the router and rout a rectangular hole in the front piece of scrap. This hole will hold the jig "finger".
Cut a piece of wood, perhaps from your board, just larger than your bit size, in my case 1/4" x 1/4" x ~2"
Fit the piece in to the slot you cut in the jig board. File or sand it down to a tight fit. Also, sand the bottom down so the finger is flush with the bottom of the jig board.
Now, realign the jig board on the router table, against the miter gauge. This time, however, you will clamp the board offset from the bit by EXACTLY the width of the bit (1/4"). I used a 1/4" drill bit as a spacer between the jig finger and the router bit. Clamp the jig board in to position. DO THIS STEP CAREFULLY. Be sure you are measuring against the widest part of the router bit and that the fit is as exact as you can get. This step determines how well the box will fit.
Now, with the jig board clamped in place, route a second hole in the front of the jig board. It should now look like the picture.
Step 3: Cut the Fingers on the First Piece
For the first cut: Align one side of the side piece with the inside edge of the jig finger. Route a hole. You should now have a piece that has a 1/4" square hole, 1/4" in from the left edge (see picture).
For the second and following cuts: move the piece so that the slot you just cut fits over the jig finger. Route a new hole. Repeat until you run out of board. If the last hole doesn't align perfectly with the edge, don't worry, the corresponding piece will have the same amount of left over hole.
Between routing holes, especially if the jig finger is a tight fit, you may want to run a square file through the newly cut hole to adjust fit. Don't file away too much, just enough for it to fit tightly.
Step 4: Cut the Fingers on the Second Piece
Take the corresponding piece you just cut, place the hole closest to what will be the top of the box on the jig finger. Align the back piece with the edge that will be the top of the box touching the top of the side piece (this is where the pencil marks come in handy).
Route a hole. This will make a hole that is at the top of the edge, corresponding to the top finger of the other piece. If everything is well aligned, the router bit should not cut anything from the piece on the jig finger.
Now proceed as before, cutting fingers by aligning the holes on the front piece on the jig finger as before.
Step 5: Test Your Fit
If the ends of the fingers are too long, that is ok since you can cut/sand them down after the box is complete. If the ends of the fingers are too short, your bit is not high enough. You might be able to file the holes slightly deeper to make a better fit.
Step 6: Make the Other Two Sides
Step 7: Bottom and Lid
To do this, I set up a fence against the router bit, now a 3/8" bit, set to a height of about 1/8". The fence is just about even with the edge of the bit. Again make a test cut or two on scrap to test the fit.
The miter gauge helps to keep the lid square when routing the short side.
Step 8: Glue and Finish
Stain or paint as you like.