I extend all the way, fill to near the top, slowly bring up the piston and very slowly extend again so that the air comes out in the fewest bubbles possible. Then wait for like 30sec or longer and do the same.

If you have small bubbles you moved the piston too rapidly.

Also note that when you're pouring fresh oil from the bottle, wait for the bubble at the tip of the bottle to escape inside the bottle. Otherwise you pour that air bubble into your damper which will take more time to pop.