Expressing help

My baby is exclusively breastfed but we wanted to start giving him the occasional bottle of expressed milk now he’s one month old.

I fed from my right side earlier and then tried to pump and after 20mins ended up with 5ml. I didn’t feed from my left and after 5-10 mins got 25ml.

I always thought you should pump right after baby has fed so there’s something left when they want to feed next. However, I felt like even if I pumped for hours I wouldn’t have got much. Any advice? Tips?

Ps I’m using a manual pump if that makes any difference.