When raising a skeleton servant, it is most important that the body of the skeleton be complete. If the skeleton is missing crucial bones, the results can be frustrating. One should only attempt to raise skeletons when you are sure that all or nearly all the bones are present.
While the magic involved in raising a skeleton will assemble the bones in the proper order, skeletons may be strengthened considerably by the addition of support on their joints. The most common are leather straps that bind the bones together more tightly. Some practitioners also drive metal spikes are between the joints, which is more expensive and time consuming, but they protect the servant where it is weakest. The details of this are unimportant as even an amateur can strengthen a skeleton significantly. Only practice will reveal the best methods of binding and reinforcing the skeletal servant. Amateurs often make the mistake of binding the bones too tightly, limiting the skeleton's movements and making it useless. Again, only practice can give the necessary experience in these matters, though it is best to err towards tight bindings. One may always loosen them at a later date.
One more note to the student: While most undead can be raised again and again, skeletons are often damaged in ways that make raising them again impossible. This is another reason that care should be given to the skeleton's preparation. Too many young Necromancers raise every skeleton they see with little or no preparation at all. Given the difficulty of obtaining corpses, this kind of inefficiency cannot be tolerated.