I have mixed it as a sensitizer for other processes. First, determine what strength you wish to make the solution. Second, determine the amount you wish to make.
If I wish to make 100ml of an 8% solution of Potassium dichromate (PD), I would add 8 grams of the PD to 100 ml of distilled water and stir. If I wanted to make 25ml of an 8% soluition, then I would use 2 grams of PD to 25 ml of distilled water. There's a formula in there somewhere.
Pretty straight forward...safety precautions...dust mask, but most I have dealt with has been more like small crystals, rather than a fine powder, with nothing going into the air. However if dropped or some other type off accident happens, you do not want to breath this stuff. Gloves -- yep, always. Label it -- looks like a sugary drink -- don't store in the fridge, near glasses of orange juice, etc.
Lasts forever once made -- store in a good bottle. Do not contaminate with organics. Label it as poison.