How to start exclusively pumping

What is the best way to start? I currently breastfeed my baby but always have to top him off with formula ( about 2 oz) because he does not get enough from me. Lately, he will only eat at my breast for a bit and then fall asleep so i'm thinking of exclusively pumping to hopefully build supply and replace the formula.           But when i pump, i usually only get about an ounce total per sitting...sometimes less. The one time he only ate off one breast, i tried to pump the other and nothing almost came out.