I have been looking at dye transfers now that the Efke brand materials are available in the US. The process for color separations is described in the Kodak publication for the dye transfer process. This publication was available as a PDF download, but I couldn't find it to give you the URL.
What you do is take a color slide and expose it to three separate sheets of black and white film using an enlarger. You can use any white light source, but an anlarger lets you control the light better. Each sheet needs to be exposed to a different filter. One is exposed using a #29 red filter, another with a #58 green filter, and the third with a #47b filter. These are then developed as normal black and white negatives. You need a densitometer to see if your exposure and development was consistent with each color in the neutral shades like black shadows and white highlights. You are supposed to plot the gamma of each to see if it matches the others. If you can find this download, it explains it in excrutiating detail.
Bear in mind that a color negative is a color separation. But another way to make these is to photograph the same scene three times but use the three filters for each respective shot. That way you bypass the color film altogether.