I made a mallet years ago; it was a block of wood with part of an old broomstick glued in as a handle (photo 2). It's functional, but pretty battered now and I'd wanted to make a proper one ever since I saw a very short set of instructions in an old Lee Valley Tools newsletter. I figured it would be a fun build and it would give me a good excuse to try out my new router table.
Not much to do design-wise really - you could do it by eye based on the sketch from the newsletter - but I enjoy tweaking things before actually building them (I generally only ever build one of every project to minimize both waste and boredom), so I mocked it up in Sketchup first (photo 3).
Step 1: Materials and tools
This project just requires a few pieces of wood, and it's a good opportunity to use up some scraps that you'd otherwise just burn as firewood. If you use a mallet a lot, build it out of hardwood. I don't anticipate wearing mine out any time soon - I'm very much a weekend woodworker - so I just used a piece of softwood. You need a saw, wood glue and some clamps. I used a miter saw and a table saw because I have them in my garage, but a regular handsaw would do the trick. I cleaned mine up with a router, but lots of sandpaper and/or a file would work fine.
Step 2: Cut
Cut out all the pieces from 18 mm (3/4") wood. None of the dimensions shown are critical, and it is easy to tweak the design to your liking once you've made it, at least as far as the length of the handle and the size of the head goes. You'll see how at the assembly stage.
Get a piece of wood 350 mm long and rip at a slight angle; I made mine 40 mm wide at one end and 30 mm at the other (i.e. the angle on each side is about 1 degree). This is easy with a table saw, but not difficult with a handsaw either. Trim the ends.
Rip another piece of wood to 90 mm wide, and cut two pieces 120 mm long and two pieces 45 mm long.
Step 3: Assemble
Place your handle exactly in the middle of one of the 120 x 90 mm pieces, so that 25 mm of the thickest end of the handle protrudes past the top of the mallet head. Now glue your 45 mm pieces on the outside of the handle (but don't glue the handle itself). Clamp, remove the handle, clean out any squeezed-out glue from where the handle will go, and leave to dry for at least 20 minutes. Glue and clamp the final layer of the sandwich on. It will look horrible; don't worry about it. The magic happens in the next step...
Step 4: Shape the head
Now that you have the head all firmly glued together, it's time to make it look nice. I made 85 degree cuts on each side to make a trapezoid and then two more at 90 degrees to the new faces across to where the handle emerges from the top. Dry fit the handle - if the mallet is too big, trim it to suit you better.
Step 5: Routing
I then ran the mallet head past a small roundover bit on my new router table, but you could do this instead with a sharp plane and/or sandpaper. I also carved a nice rounded grip with a large roundover bit, and drilled a hole in the end to make it easy to hang up. Note the end grain on the striking faces.
Step 6: Finish
Glue the handle in place if you like, but I didn't bother - it's not going anywhere. Sand and finish to your liking; I used some clear finish to stop it getting too dirty and to make the grain pop a little more. Go chisel something!
Note: there are a bunch of nice mallet instructables out there if you want something a bit different from this simple one: turned, another turned, one piece, dowel-fixed, weighted and a really high end mahogany/curly maple one.
For a superb stop-motion construction of a turned mallet, see Frank Howarth's YouTube video. Actually, do yourself a favour while you're there and check out some of his other work - amazing!