This is something that I posted a picture of a few months back on Facebook, and had many messages follow with requests for instructions, or for me to make one for people. This one is obviously a Captain America shield (at least I hope it's obvious.)
This shield is about 5cm (~2") in diameter, and could be any round shield or other emblem that you can draw. Any that aren't round could easily just be drawn with a circle around them if you needed to.
Step 1: You Will Need
1.) A soda can, small or large isn't important (as you'll see later.)
2.) Permanent markers.
3.) A file.
Optional.) Hot Glue, a magnet, & a paper-clip (for making the shield into a wearable piece.)
Step 2: Clean, and File the Bottom of the Can Off
I didn't mention the alcohol prep-pad in things you'll need, but they're always handy, and my can just had some stickiness from a spill (if you're using Sharpies like I did, then they will help with re-dos.)
File the very bottom off of the can, this doesn't take long, and doesn't really require a file. The last few I made were done by scraping the bottom of the can against my concrete driveway.
As the bottom starts to file through, there will be a very sharp, exposed edge on both sides of the split.
You'll get a feel for how it splits as you file it down, and by putting slight pressure around the edges as you hold it, you can separate the bottom away as you go.
Step 3: Pop the Bottom Out
Once it's almost filed off, you might be lucky enough that the bottom will protrude, and you'll be able to pull it out (it should snap off easily) otherwise, you can just file it off completely, and tip the can up to make it fall out.
Again, be careful as these edges are very sharp.
Step 4: Smooth the Edges
The filing will have left a very thin sliver of aluminum all the way around the piece; this can be filed off.
The sharp edge can often be snapped off if you just work it back, and forth. File the edge smooth so that it won't cut anything.
Step 5: Decorate With the Markers
Draw on your design with markers.
I have etched mine with a small screwdriver before to give me some definition between colors, and to mitigate color-bleeding in the past, but this one is just freehanded. You can use a pencil to make feint lines to follow if you need to.
That's pretty much it for the shield, but what to do with it...?
Step 6: Wear It
With only a slight modification to a paperclip, it can be hot-glued into place, and either pinned onto a garment, or bag. If it is to be worn on a shirt, or something you'd prefer not to put a hole in, the paperclip can be held in place by placing a small (strong) magnet behind the garment.
This method also makes a great way to mount this shield to a Captain America action figure if you open up the arm, and place the magnet inside.