Buy a lube pack from WASD keyboards
Attach the key the easiest way you can
Before putting the key onto the switch, insert some lube into the holes right from the lube pack
Also lube the front and back of the costar inserts, as they touch the sliders and produce sounds when not lubed
Alternatively, lube where the metal meets the slider, the metal holder part, beforehand
Alternatively, lube the middle part of the metal, as sometimes that metal part hits the key, the bent part, beforehand