Have you ever done gum printing before? If not, I would suggest starting with a monochrome copy negative and get an idea of the contrast you will need for gum printing. Gum probably works best with lower than normal contrast negatives. (That is unless you want a high-contrast posterized effect.)
The color separation filter negtives will each need different exposures and IIRC are developed to different contrast indices. That will take some fine-tuning, so if you already have a monochrome negative that prints well you should be able to gauge the look of the separation filter negatives better. The inherent high contrast of the original slide will also complicate things.
Next you will run into the problem of registration so you may want to consider this beforehand. I would suggest using a punch of some sort and registration pins on a homemade easel. You can buy standard registration pins, but I've also used wooden dowels for this sort of thing. Just be sure the pin/dowel diameter matches the punch diameter and spacing. Punch the films before exposure and with the homemade easel securely in place, expose the films once you have figured out the details.
When you have the negatives you can use the homemade easel (a flat board, maybe MDF or something, with the two pins/dowels positioned in drilled holes) to register each of the negatives on top of the paper (which has also been punched) for each of the print layer exposures.
Although this sounds like more work than using a film holder as an easel I think you will enjoy better success with the pin register method. It is difficult to register negatives visually when gum printing unless you incorporate some sort of registration mark in the film. Visually you are trying to register a negative that will probably have a relatively high value of filmbase-fog over an orange or dark-colored pigmented dichromate emulsion that has dried over the underlying printed layers. It is very tricky when doing the yellow layer first since the chroma of the emulsion interferes with seeing the yellow layer. That's why many people start with a blue layer or cyanotype layer when doing 3-color work.
The film holder guides might provide a sufficient edge masking effect to allow visual registration, but the pin method would probably work much better with conventional panchromatic copy negatives.
The difficulties involved in making color separation negatives for contact printing have long prevented me from doing 3- or 4-color gum prints. This is one area where digital negatives really make the process much less frustrating, but that's a discussion for the
http://hybridphoto.com site. Personally, I've only tried the conventional color-separation negative route with gum printing a few times and was put off by the tedium of balancing the negatives properly. But, I've done the digital negative route several times with good success. If you are interested in seeing such an example 4-color gumprint (be forewarned it will be a nude image), send me a PM and I'll provide the hyperlink.
Joe
