Not as tough as it sounds.
I do have the benefit of a Colorstar analyser, which can do a reasonable job of acting as a trasnmission densitometer, so my life is a bit easier, but I also have done this before I had the gadget. It is a lot easier to do on 120 than 35mm , but both work; I imagine 4x5 would be a breeze.
To make the mask, use the enlarger as a light source is a good start. Place the film to form the mask on the bottom, emulsion down. Place the negative to be masked on top, emulsion side up. Cover with glass to keep in contact, and register - thumb tacks into the alignment foam core or wood channel, etc. Cover the whole works with a diffuser- I use a sheet of diffusion, but a kleenex will work.
My mask film is usually ortho - either Kodak 'fine grain positive release' (35mm), or cut down lith film (any size), so I can work by safelight. I don't usually fret about unsharp on colur prints - the colour itself usually dominates the impression of a print.
I develop to a low contrast index ( say .3 of so), usually by inspection in a low contrast developer (very low contrast if I start with lith). Once dry, sandwich with the original in register in the enlarger, adjust paper contrast to suit, and print away.
Howard Bond I believe did a very detailed article in late 2005 in 2 subsequent issues of Photo Techniques that puts all the math into this rather vague description, and describes how to do it with pan films - tmax 100 I believe.