One good piece of advice I received from a lactation consultant was to not mix formula and breast milk. Her reasoning was because there are no studies that show what happens to the properties of breast milk when it is mixed with formula. So feed the breast milk separately from the formula.
I started a slow transition to formula at 4 months. Feeding 4 ounces breast milk 2 ounces formula and worked up to half and half. Now at 7 months my baby gets a full bottle of formula twice a day and breast milk in the mornings and evenings when I can breast feed.