The mesh, in most cases, is glued to the frame. Screens are labeled by mesh counts. Different mesh counts for different jobs. The higher the mesh count, the finer the detail or line you can hold with your screen and print in your design. After you have your screen you will need to apply emulsion onto its surface.
Emulsion is a photosensitive solution that reacts to UV light.
It reacts in the same way that film reacts to light when you take a picture. Emulsions can be in liquid form or in capillary film form. Most emulsion manufacturers suggest three coats squeegee side and two coats print side. If you are doing print runs under 144 items, my way works very well. It is my belief that emulsion makers are out to sell more emulsion.