How do I build a stash?

You don't need a stash necessarily.. as long as you can pump what your baby needs for the next day. Having extra is good for a vacation, or if your milk dries up

Do you have a large supply? I've been pumping one side while he eats from the other. I've also been taking fenugreek and drinking mothers milk tea to increase my supply. The more the baby is out to the breast, the more your body will make. Also, at night, are you pumping if LOnis sleeping more than 3 hours? I set an alarm and get up to pump even if my LO is sleeping more than 3 hours. That should help. 

You can also pump for fifteen minutes after he nurses. 

Pump after you feed maybe once or twice a day, and I pump in the middle of the night too. I was able to build up 300oz before going back to work and now I rotate through my supply and have been able to build it to 350 or so. Good luck!

It is time consuming, but power pumping once per day really helped me. 

I started pumping in the morning after breastfeeding my baby. I tend to because to pump the most in the mornings, which is pretty typical for a lot if women. I started freezing that milk for my return to work. Now I'm using that milk once a week while I'm at work.

How big of a stash are you trying to build up? I pump more at work than what my baby has been drinking during the day. I freeze the extra. If you can get about a days worth stored it's nice. You don't want your baby to end up eating more than you are producing.