Assembly of a wheel starts with the big three: hub, spokes, and the rim. You will need a spoke wrench, truing stand, a flathead screwdriver, and a lot of patience. Wheelbuilding is an art.
On average, wheels have 32 spokes. There's variation, but keep in mind that with fewer spokes requires a bulkier rim and hub to handle the tension spread out farther. Nickel-plated brass spoke nipples are commonly purchased with the set of spokes.
Let's get started: It's a great idea to lubricate spoke threads, holes and nipples during assembly. Having grease on these spots allow the nipples to reach high tensions (super important). After that preparation, it's time to start lacing the spokes.
NOTE: This is just one of many ways to lace spokes.
Spokes are laced in groups. The first place to start is aptly named the key spoke. If a mistake is made on the first spoke, the others will all be assembled in correctly. It goes next to the valve hole on the rim and through the hub flange, inside to outside. Spokes are set trailing the freewheel around the circle. Along the rim, spoke holes are drilled offset to each other every other hole. They are a little forward from the corresponding hub flange to accommodate the trailing style that spokes are laced. Looking at the wheel from the drive side, the spokes should go clockwise.
Loosely attach spoke nipples to hold the spokes in place. Don't tighten until al the spokes are in the rim and ready to rock. On a 32-spoke wheel, the spokes are installed in groups of 9. They should be evenly spaced, and wired to the same side on the rim as the corresponding hub flange. When the first group is in place there should be a spoke, 3 empty holes, another spoke.
Next comes the 2nd group. It should follow the holes behind the 9 first spokes in the same flow. The first spoke of the second group goes into the first hole opposite the key spoke. After this group there will be 18 spokes in the wheel, all up to this point have been trailing spokes.
The third and final group is a leading group. The spokes will insert the opposite direction and cross 3 of the trailing spokes before finding their home. Remember to keep the spokes in holes on the same side of both the rim and the hub. Install the remaining spokes this way to complete what is called a normal cross 3 pattern. Now the spokes will need to be tensioned properly.
If you are still unsure, check out the great bike tube tutorial on how to build a wheel. It really breaks it down and there's a lot of humor to keep you awake.