What size negative?
I wouldn't begin to think about spotting a 35mm negative, but spotting (or dye dodging) a 4x5 is a routine darkroom trick.
A fundamental consideration: spotting prints involves adding density to unwanted highlights - makes those highlights less visible and distracting. Spotting negatives involves adding density to shadows so that those shadows are lighter in the final print, revealing more detail.
I use a sheet of film that has been fixed out and washed. Tape this to the back of the negative. Working over a lightbox and with a magnifier, I apply a dye to the second sheet of film in areas where shadows need to be opened. I use a fine brush for small areas, and Q-tips for larger "wash" areas.
I normally use Dr. Martin's Transparent Water Color Dyes for dye dodging. I let a drop of the dye dry on a plastic palate, and then I pick up some of the dried dye using a brush dampened in distilled water that has a bit of Photo-flo added to relieve surface tension. I normally use a magenta color because I print on variable contrast paper, and in addiition to increasing the density of the negative in the spotted areas, the magenta color also acts like additional variable contrast filtration to increase local contrast. I also have some yellow dye that I can use if I want to reduce local contrast, but that seems to be far more unusual that the situations calling for magenta.
It is theoretically possible to dye dodge directly on the back of the negative, but using a second sheet of film provides two advantages, First, if I'm unhappy with the result, I simply peel that sheet off, wash it to eliminate the dye, and start over. Second, the physical separation between the dye layer and the negative consisting of two sheets of film means that if I focus on the negative, the dye will be slightly out of focus. That means that the final print will show the effect of the dodging without showing any detailed imperfections in the application of dyes.
Incidentally, it is also possible to use an ordinary pencil to dodge a larger negative. The back of most sheet films has a slight "tooth" to accept penciling.
