Key concept:
thin sheet metal vs solid or explosive projectiles.
ikar01's examples show the result of explosive projectiles which (like a grenade) turn into fragments once exploded. Cannon can use explosive projectiles whereas machine guns generally do not. Most damage from flak (anti-aircraft guns) would be from fragments coming from the exterior of the a/c unless it was a direct hit, then there would be some explosive effects (deformation in addition to scattered torn holes.)
So to get more basic, before you can depict 'battle damage', you should consider the source of the damage: AA or other aircraft, projectile (solid shot - heavy or lighter - or fragments), or explosive (subset direct or indirect). Consider the path of the projectiles or fragments and how that would impact the holes in the skin. Even figure in whether there might have been impact damage from "stuff" (or even other airplanes!) hitting your a/c.
Also, don't forget that not all parts of the exterior skin will be metal. Fabric covered control surfaces were common in WWII (WWI being of course mostly fabric covered...) while the 'jet age' forced a change to almost all being metal covered. Plus, there are parts made of other materials to be addressed such as fiberglass, plastics, etc. - so know the materials of your subject.
Then, as Chris recommended, look at a selection of battle damage images, the more the better. Note that sometimes there is fire damage as well, which also impacts the situation since most metals will weaken under high heat and deform under the slipstream & structural loads. Speaking of structure, you might wish to also depict some internal damage as well since it may be visible through the 'holes'. Think what the result was after the 'hit', including what did the airstream do (like tearing away portions of a fabric covered surface exposing the underlying metal framework), what components were damaged (such as was there a fire after an engine hit...)
As for the actual damage, you should aim for a 'thin skin' then rip it & tear it according to your ideas of how the damage came about. Cutting out a section and replacing with a bit of aluminum foil - blended in at the edges with putty - looks very good for larger, exploded or fragged sections. Smaller areas can be thinned from the inside and then punched with a needle, knife tip, whatever does the job. Glass will shatter & crack, plastics will crack, break off in chunks or show rough holes, fabrics will tear & then shred into tatters in the wind or just have relatively clean holes punched through (can be depicted using thin paper applied over a plastic card framework... paint carefully with thinned 'white glue' to stiffen, then 'rip' away.) Even the classic 'movie' effect of the bullets 'stitching' across the airframe could be done, though it would tend to be much more random than regular! (You've seen those WWII gunnery films used in so many movies that show how the tracers tend to go arcing along in rather less than orderly fashion?) Aerial gunnery isn't like a laser beam but more like a spitting waterhose. Both a/c are moving, the guns firing shakes things up and there is the offset from the 'synched' aiming point. If you took some paint and then flicked it off a toothbrush onto some paper, you'd get a bit of the pattern involved...
Although you asked a simple question, there are a lot variables involved. Nice thing is, you can practice your technique(s) before committing to your model!
Hope this helps; Regards, Robert