The below method is great for separating a file for conventional production, it splits an existing tonal layer.

1. Have your masks or selections done.
2. Use retone or continuous tone depending on having a mask or a selection.
   
3. For each colour you want to split, duplicate the layer to the number of colours you want.
   
4. Make the 1st layer active.
5. Pipette the colour you want to be the base colour and write down the percentage by checking it in the Info Window.
   
6. Drag the colour from the tools window to the relevant layer.
   
7. Go to Edit Menu > Gamma and pull the value noted on step 5 to 100%.
   
8. Apply Gamma.
9. Make the 2nd layer active.
10. Pipette the colour you want to be the medium colour and write down the percentage.
    
11. Drag the colour from the tools window to the relevant layer.
12. Go to Edit -> Gamma and pull the value noted on step 9 to 100%.
13. On the same Gamma window pull the value noted on step 5 to 0%.
    
14. Apply Gamma.
15. Make the 3rd layer active.
16. Pipette the strongest, most saturated colour on that layer (usually 100%, but it might be less in some cases) and write the percentage down.
17. Drag the colour from the tools window to the relevant layer.
18. Go to Edit -> Gamma and pull the value noted on step 16 to 100%
19. On the same Gamma window pull the value noted on step 9 to 0%.
    
20. Make 1st layer active.
21. Go to Separate -> Combine to layer.
22. Click on Make.
23. Set 1st layer to 100%.
24. Set 3rd layer to 0%.
25. Click Fix Layer.