I have never made an internegative from a Scala original. However, I have made many from colour transparancies over the past ten years. The procedure should be the same, with possibly a small adjustment required in development, as Scala uses silver to form the image, and colour transparancies use dye.(posted 8881 days ago)Basically, the procedure is to project your slide on to a sheet of B&W negative film, and to develop it. The slide should be properly exposed, with adequate detail in the shadows. High contrast slides will be a bit harder to obtain a decent final print from, but it can be done.
You can also make the interneg by contact, but dust will become an issue, a major issue. Also, any defects in your original will show on the interneg and the print. By defects I mean scratches, abrasions, watermarks, fingerprints and other physical shortcomings. On the positive side (pardon the pun), the interneg will be just about as sharp as you can make it.
If you choose to make your interneg by enlargement, use a diffusion enlarger. A condenser enlarger will render detail better, but will also give you problems with physical defects, although it won't be as bad as by contact. Use a lens of adequate focal length to cover your original, and try to use an aperature about half-way in the lens's range.
Mask the transparancy so that there is as little stray light as possible. If you are printing from a 35mm original, remove it from the mount. You may wish to use a glass negative carrier. On one hand, the trans won't be able to "pop" or shift in the carrier and cause an out of focus image; on the other hand, dust can become a problem, as well as Newton's rings. I prefer a glass carrier, but you will have to find what works the best for you.
Place the transparancy in the negative carrier emulsion UP; that way, the internegative can be enlarged or printed by contact without the image being laterally reversed.
I use an ordinary 4x5 film holder to expose the film. On side has a sheet of white paper for composing, and I focus on the other side on a sheet of scrap film which I remove before inserting an unexposed sheet in the dark. Use a focus magnifier for the sharpest images.
I used to use Kodak Super-XX film, which is no longer available. I now use Ilford FP-4 Plus developed in Kodak HC-110, dillution D, and have used Kodak Tri-X Pan Professional, developed in D-23, in the past. I develop the FP-4 for 5 minutes in a tank at 68:F with agitation at one-minute intervals; for tray development with continuous agitation, try about 25% less time.
For exposure, I use an ordinary studio meter. First I compose the image; then, I remove the negative carrier from my enlarger. I set the ISO speed to 32 and take an incident reading where the interneg film will be exposed. With the enlarger lens set wide open (usually), I obtain a reading of f/2.8 @ 1 second. Then, with a combination of f-stops and neutral density filters, I close down 7 stops, and expose my film for 5 seconds.
Sorry, that was rather long-winded! Give it a try; after a few tests you should be able to produce decent internegs.