I recommend you give the reversal method a try, as outlined in my article Less is More at http://unblinkingeye.com/Articles/NbyR/nbyr.html. I have found this to be a very cheap and effective method, with many fewer steps than the interpositive method.

There is also the traditional interpositive method, as evinced in Bob Herbst's article Enlarged Negatives using APHS Ortho Film and Pyro Developer at http://unblinkingeye.com/Articles/EnlargeN...enlargeneg.html. This is probably the ultimate way to go, but it is much more time consuming and expensive. He sent me an original 4x5 negative, the enlarged 8x10 interpositive, and the final 8x10 copy negative he made from it. I was astounded at the precision and perfection of his work. All negatives were absolutely free of dust and finger marks. It was obvious to me that he is a meticulous worker who has very carefully worked out the details of the process.

There is another interesting technique that I'm still experimenting with. An article by William L. Jolly in the Jan/Feb 1992 issue of Darkroom & Creative Camera Techniques describes making direct positive transparencies (effectively, interpositives) using Kodalith or any standard film by giving it a flash exposure part way through development. This is essentially a form of solarization (the Sabatier effect), but it can be controlled in such a way that the flash exposure produces very subtle enhancements of various portions of the positive image. For instance, dull greys can be made to appear white, or you can obtain total sabatier reversal of some areas if you desire. You can then contact a negative from this positive. The technique has a multitude of creative possibilities, and I'll probably write an article about it in the next six months or so. So far, I've only done some preliminary experiments to verify that I can get sabatier reversal with the APH ortho/litho film. There may be other films that work a lot better, but the nice thing about the ortho film is you can use it with a red safelight, so you don't have to work in total darkness.