A serving tool is used to quickly serve bowstrings. Although commercial versions are available at reasonable prices, making on should not take more than an hour or two. Also, commercial serving tools are probably(?) designed for relatively thin serving thread and bowstrings: this in itself may be good enough a reason to make one.
Process step by step Edit
First choose a piece of wood for the use as the drum. Mark the center on both sides. Unlike shown here, starting with a rectangular piece is probably easiest, as marking the center is easier:
Next drill all the way through the drum:
Push a bolt of proper size through the drum and tighten it with a nut:
Next attach the whole package to a (bench) drill and use a combination of rasps and files to "turn" the drum:
Don't use of any edged tools (chisels, knives) or you'll probably get hurt. Also avoid excessive pressure on the drum, or it will become oval instead of round; it's better to set the drill to high speed and apply less pressure. The end result should look something like this:
At this point make sure that the hole and the ends of the drum are exactly perpendicular. If they're not, the force required to rotate the drum varies, as it tries to force the frame (see below) outwards. This means that the serving will be somewhat loose in other parts and too tight in others. In worst case, the serving thread will break every now and then due to excessive pressure.
Next you need to make the frame. Thin strip of steel will work fine. First drill a hole for the serving thread to the middle of the strip:
Then bend the strip to proper curvature and drill holes for the bolt going through the drum. Finally round all corners of the serving thread hole carefully, so that the edges don't cut the thread during serving. The end result should look like this:
Notice the concavity in the center of the strip: that's the part contacting the bowstring, so make it needs to be wide enough to rotate freely around it.