Formula Milk & Breast Milk

As my breast milk is still insufficent, i nd to supplement w fm. So do i mix both bm & fm together n feed e baby or i feed baby w bm first followed by fm separately? And if my baby has been drinking total volume 70ml fm, and i have 20ml bm. So i prepared 50ml fm and count in e 20ml bm so tat e baby is taking total 70ml of milk?