You'll need some wood glue, 3/4 inch stock, 1/4 inch birch plywood, 3/8 inch spruce plywood, 1/2 inch dowel and 1/4 inch dowel. For tools, a drill press, router table, disc sander and scroll saw or bandsaw are all useful. But you could get by with a fret saw, sand paper, rubber bands and hand drill.
You can see a 3D model of it at my Thingiverse account.
I printed the gear templates from Woodgears.ca.
Step 1: The handle end
Cut the handle out of 3/4 inch stock, using the PDF provided. You may want to ignore the drilled holes in the PDF, and instead base your holes on your completed and sanded gears, making sure they rotate freely. Use a router and a roundover bit to make it more comfortable. I avoided rounding over near the 1/2 hole, as I was afraid of tearout. Round it over before you drill the holes and you won't have this problem!
Adhere the gear templates from the PDF provided to some 1/4 inch (approximately) birch plywood. You might get away with spruce plywood, but it's generally not a good idea for gears, as it doesn't wear well. If you don't know what your plywood is, a good rule of thumb for this width is: if it has 5 ply, you're good, if 3, not so much.
Using an awl, eyeball and mark the centre of each bottom land. Use the awl marks to drill out the bottom land. This is much easier than using a saw to do it, and the extra material that is removed doesn't affect anything. Of course, you're welcome to use a saw for this if you're more patient than I am.
Roughly cut your gears out, leaving about a 1/8 inch clearance around the outside. Next, cut the teeth. You'll have to cut them on a 10 degree angle. If your bandsaw or scrollsaw doesn't incline 10 degrees in both directions, clamp a temporary platform with a scrap piece of stock under one side to make the angle. Put your saw back to 90 vertical and cut the tip of each tooth.
Use a sanding block to lightly round each tooth point, and clean up any imperfections.
Use an awl to mark each gear centre. Drill the center of each gear to accept 1/4 inch dowel. The large gear hold should be slightly oversized. The small gear centre should be slightly undersized.
You need 5 small circles. You could make them on a scrollsaw or bandsaw using a jig, or a plug cutter on the drill press. I chose the following method. Each is about 3/4 or 7/8 inch in diameter with an undersized 1/4 inch hole. Two are made from 1/4 birch plywood, one from 1/8 inch plywood from a box of clementines, one from 3/8 inch spruce plywood and one from 3/4 inch scrap from the handle (this only has a hole drilled halfway through).
Mark your circles on your stock. Drill an undersized 1/4 inch hole in each centre. Roughly cut out your circles.
Take a small 1/2 inch or thicker board and drill an undersized 1/4 inch hole very near the edge (a drill press is recommended to keep it very straight). Insert a dowel and cut it off, leaving 1/2 inch or so above the surface of the board. Get two clamps that you can use one-handed. Position the jig on the sanding disc's table so the dowel is about 3/4 inch from the disc. Clamp the back of the jig to the table loosely. Place a rough-cut circle on the dowel, and, holding it down, rotate the jig into the moving sanding disc until it's almost to your line. Holding the circle and jig firmly, use your other hand to clamp the front, and then tighten the back. Rotate the circle slowly to sand smooth, using a firm, downward pressure with your index finger, and rotating it with your thumb and middle finger. Repeat to get exactly to the line.
Putting it all together
Dry fit everything first, and make sure nothing interferes with the movement of anything else. The washers shown in these pictures were ultimately not used.
Glue the 3/4 inch circle to small piece of dowel. Put that dowel through the off-centre hole in the large gear and glue the 1/8 inch circle on the back. You don't want this tight, nor super loose. Allow to dry, rotating it carefully every few minutes so as not to glue it to the large gear. Cut the dowel flush to the circle.
Round the end of a long piece of 1/4 inch dowel. Dry fit the small circle, then the small gear on a long piece of dowel so that the dowel goes to the bottom of the hole in the handle, leaving a small gap between the gear and handle. The small circle allows more gluing area, as this gear can be under strain
Assemble the 3/8 inch spruce spacing circle, dowel, large gear and 1/4 inch birch circle. You'll need to increase the depth of the 3/8 inch spacer by adding another circle onto it, or sand it to a slight angle. The idea is to get it to meet the small gear at the right place without binding. I angled my spacer, but not the dowel, so I have more slop than I'd like, but it's within acceptable limits. You're probably better off increasing the depth of the spacer and keeping everything 90 degrees.
When you're happy, glue and clamp it all up. The large gear rotates around its dowel. The three pieces of the knob assembly are glued together and rotate in the large gear's off-centre hole. The small gear assembly and shaft rotate as one piece inside the handle.
Glue the 1/2 inch dowel into the handle. Its job is to support the marshmallow end.
Step 2: The marshmallow end
Cut two full copies of the gear case front/back and one extra copy of just the top from 3/8 inch spruce plywood, or similar thickness material, using the template PDF. I'm not 100% happy with the look of the plywood. If I were to do it again, I'd use thin hardwood. Stack and clamp the pieces, with a bit of scrap 3/8 inch ply between the front and back bottoms. Drill a slightly undersize 1/2 hole and three slightly oversize 1/4 inch holes through the stack.
Cut three gears out of 1/4 inch birch, using the PDF template and method described previously, except there's no need to cut them at a 10 degree angle.
Cut three 1 inch diameter circles out of 3/4 inch stock using the method described previously, only this time, drill the centre of the rough circle slightly smaller than the diameter of the skewers you'll use. Then drill a slightly undersize 1/4 inch hole about halfway through. Then complete the circles on a disc sander. The skewers will friction fit into the ends of the circles.
Putting it all together
Cut your dowels on the handle to length. Start long, then test fit your marshmallow end and get a feel for weight. The shorter you make your dowels, the less strain on your hand (as the weight of the marshmallow end is leveraging against your hand/wrist), but the closer you need to be to the fire.
Dry fit everything, making sure the gears don't bind and there's as little friction as possible. With a 2:1 gear ratio, it wouldn't be hard to snap the 1/4 inch shaft. I can rotate mine with almost no effort. Be sure to leave a small gap between the circles and gear case.
Glue the rear gear case and tab together and glue onto the 1/2 inch dowel. Add a washer onto the 1/4 inch shaft and glue the middle gear in place. Glue the two outer gears onto their dowels. Let that dry, making sure you can rotate the middle gear occasionally to prevent accidental gluing to the rear gear case. Add washers to the two outer gears and put them in place. Put washers on all three gear shafts, then glue and clamp the gear case front (be sure to get it the right way around, to ensure the holes are lined up with the gear case's back holes). Check again to make sure everything turns freely. When that's dry, glue on the circles. I didn't end up needing the three washers between the circles and gear case.
Congratulations, you now have a working, homemade, rotating, 3-marshmallow toasting stick! If you make one, I'd love to see a picture in the comments. Please consider voting for this entry in the Woodworking contest!