Generally, there are two methods of attaching hair to wigs. The first and oldest is to weave the root ends of the hair onto a warp of three silk threads to form a sort of fringe called a "weft". The wefts are then sewn to a foundation made of net or other material. In modern times, the wefts can also be made with a specially adapted sewing machine, reducing the amount of hand labour involved. In the 19th century another method came into use. A small hook called a "ventilating needle", similar to the tambour hooks used for decorating fabric with chain-stitch embroidery at that period, is used to knot a few strands of hair at a time directly to a suitable foundation material. This newer method produces a lighter and more natural looking wig. High quality custom wigs, and those used for film and theatrical productions are usually done this way. It is also possible to combine the two techniques, using weft for the main part of the wig and ventilating hair at the edges and partings to give a fine finish.

Measurement. Making custom wigs starts with measuring the subject's head. The natural hair is arranged in flat curls against the head as the various measurements are taken. It is often helpful to make a pattern from layers of transparent adhesive tape applied over a piece of plastic wrap, on which the natural hairline can be traced accurately. These measurements are then transferred to the "block", a wooden or cork-stuffed canvas form the same size and shape as the client's head.

Foundation. Depending on the style of the wig(cosplay wigs or fashion wigs, straight or curly, etc.), a foundation is made of net or other material, different sizes and textures of mesh being used for different parts of the wig. The edges and other places might be trimmed and reinforced with a narrow ribbon called "galloon". Sometimes flesh colored silk or synthetic material is applied where it will show through the hair at crown and partings, and small bones or elastic are inserted to make the wig fit securely. Theatrical, and some fine custom wigs have a fine, flesh colored net called "hair lace" at the front which is very inconspicuous in wear and allows the hair to look as if it is coming directly from the skin underneath. These are usually referred to as "lace front wigs".
Source: http://en.wikipedia.org/wiki/Wig#Notable_wig_designers