Cover it with gauze:
This will keep the blister skin intact and let it heal on its own. The natural process includes reabsorbing the fluid and the skin will return to normal. If you are using band aid, use the ones with pores that will let the skin breath which helps in quick repair.
If the blister is large and the serum is a source of pain for you, drain it by a slight brick on one side of the blister. In any case keep the outer skin attached. Wash your hands with an antibacterial soap before starting. The pin that you use must be sterilized. One way to do it is to heat the tip on the stove till it glows red and then allowing it to cool before you use it. Cover the punctured blister with a non-sticky bandage (band aid).
Use an antibiotic ointment:
Using an antibiotic ointment is going to help it heal faster and also prevent any infection. These antibiotic creams can usually be bought at the local pharmacy without any doctor's prescription. However, in cases of severe blister, do consult a doctor before applying any ointment.