Solvent costing

Can any one tell me the steps involved in setting up co Products.I also have a scenario where in Pharma Industry where Solvents needs to be costed.Solvents are co products and are used in the next process as a raw material.SO i Introduce produce Material A and use Solvents B & C.Say 40 % of solvent is recovered goes through more processes and is added back after purifying back as raw material.