First make sure you are already getting the appropriate stimulation for your baby's age. If your nursling is under 6 months, you should be nursing and/or pumping at least 8 times per day while using a quality double electric pump such as the Hygeia EnJoye or Ameda Purely Yours or Ameda Platinum breast pump for a full 10-15 minutes per session. If your nursing is 6-12 months, you should be stimulated at least 5 times per day. For over 12 months at least 2-3 times per day.
If you are already following the above protocols for sufficient stimulation of the breast, you will need to increase the number of pumping sessions to increase milk supply using a breast pump. Research indicates that you need an hour of additional stimulation to increase milk supply. Nature dictates that the average growth spurt where a baby is increasing their nursing sessions lasts about 5 days. taking these two things into consideration, you should add 3 double pumping sessions of at least 10 minutes for 5 days in a row. Each breast is stimulated for 30 minutes totaling the 1 hour of stimulation you need. For example, if you are nursing/pumping 8 times a day, you need to increase it to 11-12 times per day. If you are nursing/pumping 5 times a day, you need to increase it to 8.
Growth spurts are just that, spurts. You need to increase your pumping sessions for a full 5 days in a row for this to be effective. If life interrupts that, you will need to start over and do another 5 days in a row to simulate the growth spurt.
You can also use breastfeeding supplements during this time for additional increase if you prefer.