What's the best way to Knock out the breather Holes?

They are made to be knocked out. I used a ball peen hammer and hit it with a brass hammer from the top down and they popped out. Then I cleaned up the hole with a rat tail file.
I did it to a pair of the same covers I purchased new and sent off and had Leanna powder coat, I did it to them after I got them back.

Yep, I use the ball end of a ball pein hammer. Smack the flat face with a brass or soft faced dead blow hammer works best. toolman