You can express your breast milk manually by using your hands or with a manual or electrical breast pump. 

How to express breast milk with a breast pump?

• Wash hands thoroughly before expressing milk. 
• Ensure all the equipment are clean and sterilised. Mark the collection date on the storage container.
• Use your pump correctly, starting with low pressure and switching between breasts after about 20 minutes of expressing milk.
• Express your breast milk every three hours and store the breast milk in the refrigerator or freezer.
  Tip: Do not store your breast milk at the door of the refrigerator or freezer, try to store your breastmilk as far in the refrigerator or freezer as possible to  prevent your breast milk from temperature changes from the door opening and closing.
  
  

Reference: Healthhub