Situate the external pond pump so its inlet is beneath the pond surface. You can achieve this by placing the inlet on the side of the pond. This ensures the pump is primed by the pressure of the water above it.
Pump the primer button to bring water into the pump. This button is present only on smaller external pumps.
Loosen the screw holding the primer cap in place on top of an external pump, using a screwdriver. Remove the cap. Pour 1 to 2 gallons of pond water into the pump, until the water level is just beneath the top of the pump. Replace the primer cap and secure it with the screw. Turn on the pump.
Install a pond priming basket just in front of the pump inlet. The basket works in the same manner as a primer built into a pump, but it's easier to access. Remove the top of the basket and fill it with 1 to 2 gallons from the pond. Start the pump.