There are many women who successfully combine breastfeeding with supplementary feeding, and continue in this way for many months. Supplements are usually given after the breastfeed. Some mothers find they only have to give one or two small supplements a day. Other mothers find they need to give one after every breastfeed. It depends on your breast milk production.
Mothers usually find they plateau-out at a certain level of supplements.
But to avoid this potential problem, supplements can easily be given to the baby by cup. Any small cup with a smooth lip or edge will be suitable e.g. a medicine measure. The baby is held in a semi-upright position, with his hands gently restrained. Rest the cup on the baby's lower lip, and dribble the milk just into the front of the baby's mouth. The baby will soon learn to lap or suck the milk.