You get some correct thickness alluminuim alloy sheet - most of the body is not structural so you can use a softer/annealed grade that's easy to work. You want the same thickness, its about 2mm. There are several ways to do it depending on where it is - how much it shows and how much space you have to work, (and how much time you ahve!)
1 Put a larger patch over the top, chamfer the edges. Make it 1" bigger all round and put a row of pop rivets in at 1" pitch 1/2" from the edge. Round the corners. If you use 1/8" countersunk rivets and countersink them just right (its takes practice) they are nearly invisible. You need to get the rivet lengh right, I tend to buy several lenghts as they are cheap.
2 Cut out the damage to an oblong (make patch and mark round that) and make a plate 1" bigger. Drill the plate 1" pitch 1/2" in all round then put it over the hole (mark out carefully so its central) then drill through. Now turn it side on and put it through the hole then rivet it so its on the inside. Then take the patch and rivet it onto the back plate. This will make a flush patch that's close to invisible if you use csk rivets. The patch should be 0.010 smaller all round so the trick is to make the patch first then use a sharp scriber and cut to the line, it will be about right. This looks great but is a lot of work, its a lot of careful fileing.
3 Make a patch, drill 1/4 in at 1" pitch, draw a line round it, Cut out 1/2" smaller all round, joggle the edges then fix the patch in with no-nails then drill through and rivet. Its 1/2 overlap becuase that's all the jogglers do. This is quite quick if you have a joggler but joggleing corners is tricky and you may have to chamfer them.
Go to your nearest aircraft museum and look at any wartime aricraft, they are usually covered in patches (they would rivet over bullet holes over night) and you can see how its done and get some ideas.
I've been on modern airliners and seen a chamfered edge (not flush) patch so they must be good for 600mph (plenty of margin there!).
Buy a set of Cleco clips and pliers - not expensive and save a lot of time.