1) spread things out in a comfortable, clean work area - you want to keep distractions to a minimum
2) there are 36 spokes for the front wheel and 36 for the rear; all the front spokes are the same length and all the rear spokes are the same length, but half of them have a slightly different angle as seen in the 3rd picture below - the spoke on the right is slightly larger than 90 degrees (90+), while the spoke on the left is about exactly 90 degrees
3) lay the spokes out for the front (or rear) and set the others aside so you don't get them mixed up; divide them into two piles - 18 of 90 degree angle and 18 of 90+ angle
4) take the wheel hub and start with a 90+ spoke and insert it from the outside of the hub (pic 4 below); go to the next hole and insert a 90 spoke from the inside (pic 5); now go around the hub alternating 90+ and 90 spokes until you have completed one side (pic 6 & 7); now turn it over and do the same for the other side of the hub until you have all 36 spokes threaded (pic 8 & 9)
5) where you start threading on the second side of the hub DOES matter because of the way the holes are laid out on the rim; you have a 50/50 chance of getting it right, OR you can thread the first side then line them up with the holes on the rim to get the correct starting point for the second side; it's hard to explain - seeing the rim layout will help
6) each adjacent pair of spokes has an over-under pattern that cross with each other, and the 90 spoke goes over and is always on top; I laid it out the best I could in pic 10
7) now take the rim and start threading the spokes into the correct holes; each crossing pair of spokes on the hub is 6 holes from each other on the rim, and the angle of the hole in the rim will make it plain
8) lightly thread a nipple on each spoke to keep it in place; once you have one side completed (pic 11), turn it over and do the other side (pic 12); when you're done they should look like pics 13-15