There are several ways of making masks. The simplest is to overlay the camera negative with clear acetate and block out the areas that you don't want exposure through with opaque. This requires some type of registration...the simplest would be to mark the camera negative boundaries with red lith tape on the negative carrier so that the mask can then be position in the same location for fairly precise burning of the areas that you wish. The same thing could be done by making a dodge mask that would be sandwiched with the camera negative for a portion of the exposure. Again the lith tape will position the negative/mask fairly accurately. Without a registration system it will not be as precise as the method that follows. However it will enable burning and dodging more precisely then not using masks.
Originally Posted by Aurore
The method that I use is to make a photographic sharp mask with lithographic film by contact printing the camera negative in register with the lith film then developing the lith film in paper developer or AB developer to arrive at a high contrast and sharp postive of my camera negative. This will work as a dodge mask by sandwiching it with the camera negative for a portion of the print exposure time. By taking this another step and contact printing the high contrast sharp positive mask with another sheet of unexposed lith film I make a high contrast sharp negative image of my camera negative and this will work as a burn mask by either using the mask alone or for very precise printing in of details in a sandwich with the camera negative in a second exposure. Typically I follow this with an unsharp mask to blend demarcation lines and to increase apparent sharpness by "edge effects". This is very precise if you have a registration system for your enlarger/negative holder/punch and mask printing frame. Hope this answers your questions.