How much developer to use when dying hair??

i'm want to dye my hair a blue/black color... i bought l'oreal preference permanent haircolor onyx sheen blue black and was told at sally's to also buy the l'oreal oreor creme volume 20 to use with it... came home and umed there was directions on the bottle but no such luck... can anyone help with how much of what... was also told i had to buy two bottles of the hair dye because of the length of my hair!! Please help!!