Probably a common question but for mamas that are exclusively pumping

When did you start freezing a stash? Or basically how did you do it?
I breastfed with my daughter then after a month she only wanted the bottle. So I started pumping but I was literally pumping everytime she ate because I would run out. I got tired of having to pump all the time so I put her on formula. Now I'm on my second so I want to be prepared or actually know what I'm doing to at least successfully pump.