How do I start pumping to build a stash?

Beener0514

I know this seems like an obvious question but I'm having the hardest time with it.

At first, I had oversupply and a forceful letdown. I did blockfeeding and it helped regulate that. I am finally at a point where my supply is (nearly) perfect for LO.

I'm now needing to start stockpiling some milk to freeze so that when I have someone else watch her, BM is available. When do I fit in the pumping session?  I've tried pumping after she's done nursing but there's nothing left in the boob. Today, she nursed on right and then fell asleep so I pumped on left. Sure enough, an hour later, she was hungry again but I had no milk left in either boob to nurse her with!

What am I doing wrong? What worked for you?