My old pair is quite sloppy (though not bad for a 10-year-old, I'd say. Good job, former me), so I decided to make a new and improved set--documented here on Instructables.
Now for the important part: what Napier's Bones are. They are a multiplication tool/math toy that allows you to multiply 0-9 with large numbers extremely quickly with only slabs of wood. As long as you add small numbers and read correctly, you will get the correct solution every time.
Step 2: Materials
Wood. Unless you have a table saw to cut wood lengthwise, your best bet will be to get 1" x 1" or 3/4" x 3/4" poles. You will need at least 30 inches of wood, but more likely around 50 inches if you want the ability to mulitply more numbers. The wood needs to be square.
Hardboard. Also called Masonite, you need a piece 10 inches high by approximately 4 inches. I got a 10 x 14 sheet and cut it in one direction.
Wood Glue, and a clamp to keep pressure while it dries.
Sharpie Markers. If you want a different look, you can use some other writing/painting instrument.
A saw (I used circular) and sandpaper.