When a decal is applied over a hole or opening, like, say, the red decals applied over the Spitfire leading edge gun openings...

...some cracking or shattering might be locally experienced when trying to pierce-open the underlying hole (or any kind of underlying opening, like an underwing shell ejector chute).

What I do is dip a toothpick into decal softener...

...then carefully pierce through the opening... The softener moistened tip will gently soften the decal locally, so it can be pierced without cracking...

Thanks my friend @chinesegeorge! Your technique sounds great, too!
I used to tear off the transfers' edges around the holes upon "working" my micro drill or my blade tip in the hole/opening, the result looking frustrating.

Thanks my friend @johnb!

Thanks my friend @brithebuilder!

Thanks my friend @bikequeen!