Drill a row of 6 3/16" holes in a piece of plastic, 1 cm apart from each other in a straight line. Use a metal guide to help position the holes correctly. You'll want to use a thin piece of plastic: a CD case works well. Cut out the part of the plastic with the holes in it, so you end up with a piece 1.8 x 6.7 cm. Round off the corners if desired. This will be one end of a bobbin. You will need two bobbins with two ends each, so repeat the process three times.
Drill a row of 6 3/16" holes again, this time through a piece of wood, for the bobbin core. Shave the wood down until it is 7 mm thick, and cut out the core, giving it the dimensions 0.7 x 5.7 cm. Repeat this process to make a second core.
Assemble the bobbins with superglue. Each bobbin should consist of the two end pieces with the core between them, positioned so that the holes on all three parts are aligned. Stick six small metal rods or screws through the holes of one of the bobbins. Cut them down so they are flush with the end of the bobbin on one end, and stick out as far as the thickness of the bar magnet you intend to use on the other.
Wind copper wire around both bobbins. The number of times to do this will vary, depending on how much DC resistance you want. A good default if you're unsure is 5,000 windings for a bridge pickup, and 4,500 for a neck pickup. This will obviously take a long time if you're doing it by hand, so you might want to invest in an automatic winder, or construct a winder yourself.
Wrap the coils in non-sticky teflon tape, and then in electrical tape. Soak the bobbins in hot wax (not more than 70 degrees centigrade) for 20 minutes to protect the coils and prevent any microphonic pickup effects.
Run screws through the open-holed bobbin's holes, and attach a pole piece holder on the other end, to make it adjustable. This holder is a small piece of metal as thick as your magnet, with holes bored through it that align with those of the bobbin.
Place the two bobbins side-by-side, with the magnet holding them together. The magnet should fit flush between the hole piece holder and the rods sticking out of the other bobbin, with the north pole facing the former and the south pole the latter. You may have to trim the hole piece holder for a good fit.
Set the humbucker into a brass pickup cover (you can cut one yourself, to fit snugly). Attach lead-out wires for amplification: see Resources 1 for different ways of doing this. Fill the pickup with hot wax for further protection, and attach it to a brass baseplate (see Resources 1 for detailed patterns).