There seems to be a lot of interest in making enlarged negatives for contact printing on Azo or for alt-processes, and much discussion of digital enlarged negatives. While Photoshop offers many creative possibilities for enlarged negatives, I get the sense that some people aren't really aware of the possibility and attractions of making traditional enlarged negatives, and it would be good if the folks on APUG who do this or who have done it in the past can share some wisdom and experience here.
There are a few basic approaches.
One is to make an interpositive with an enlarger or large format camera using a low contrast copy film or a relatively neutral film like T-Max 100, and then contact print the interpositive to the same or a similar film to make the enlarged internegative. This method only requires an enlarger, a method of contact printing, and trays or tubes to develop the film.
I believe that Weston's method was to dupe his 3x4 negs using the 8x10" camera and then contact print the interpositive to film to make the interneg, and then contact print the interneg to silver chloride paper. Of course if you don't have an 8x10" camera or you want a larger final print, then you use an enlarger, which gives you more flexibility in terms of final print size.
Another technique, if you have a sufficiently large camera, is to make a regular enlargement--applying all the printing controls at your disposal, and then dupe the print with a large format camera of the format you want to contact print.
Either the interpositive or internegative or original negative can be retouched or masked for contrast or sharpness along the way, so this process offers a lot of control.
Another possibility, which Mortenson and some of the Pictorialists used, and is really easy for most people who already do their own darkroom work, is to make paper interpositive and internegatives. One of the particular attractions of this method is that you can easily retouch the back of the interneg or interpositive on a light table using soft lead pencil or charcoal or a drafting pen for sharp lines, etc. Apply pencil to the interneg to push up highlights and to the interpos to deepen shadows. Neat, eh?