Sure it is no real secret. The key to this pattern lies in 3 separate genes. You need mo (mottling) Pg (Patterning gene) and Dilute. It will take quite a while but the process is as follows.
First you need to breed a bird that has pure mottling into a partridge bird that is pure for the Pg patterning gene(preferably an e^b) based bird. Once you have a line of partridges carrying mottled then you have two options to enhance the colour. You can either add in Dilute (Di) as i have in the bird above, or Mahogany (Mh). Dilute will act to enhance the pattern ina lighter shade, wheras mahogany will give a richer red colour to the line.
If that is too complex, then simply cross a good black mottled bird to a good partridge bird, breed the offspring together, and keep breeding back to the offspring that look the most like a partridge. Because the mottling gene is recessive it will stay in the line.
Feel free to drop me a message or pm if you need anything explained further, i am happy to help where i can