How to latch baby? And produce more then a oz each pumping session?

Tips on increasing milk supply? Second time mom here my first I tried to breastfeed but it didn’t last long because I wasn’t making enough milk literally only little then a oz each pumping session,  I’m not sure if it’s because my son’s first three feedings right after birth had to be formula due to low blood sugar. Or if it was the wrong flange size. Or not eating properly or right foods. I’m a bigger person so I’m not sure if my son was latched on properly! Lactation lady was rude in the hospital. She told me breastfeeding won’t work because my breast sags. My C-section is scheduled in 17 days I am determined to make this breast-feeding journey work this time!!! I have different flange sizes just ordered this wearable pump. Does anyone have this pump? If so what is your likes and dislikes about it?