On Double Transfer
A "temporary support paper" is used to combine the three separate color layers . This is a thin, high wet strength paper that has been coated with unhardened gelatin. First, the temporary support paper and the cyan layer are soaked in cold water and combined. When dry, the plastic support will peel off the paper leaving the cyan pigment image on the temporary support. Next, the temporary support is re-soaked along with the magenta layer and combined in register (this is the difficult part). When dried, the temporary support paper, which now contains the cyan and magenta image layers, is soaked again along with the Yellow layer and combined, registered and dried as before.
A sheet of "Final Support Paper", coated with a thick, hardened layer of gelatin, is then soaked in hot water and combined with the temporary support paper (which has once more been soaked in cold water). Allowed to dry for 10-30 minutes, the sandwich is then placed in warm water and the temporary support is peeled away leaving a beautiful multi-color carbon pigment print.
All this can be found in many books, but it will be necessary to modify the details of time, temperature and technique to reflect the unique characteristics of the materials being used.