I don't play any string instruments, I did this pick for a friend who plays the bass, I guess this kind of picks should be used with instruments that require high pressure on the cords in order to play, I can't give additional information about the ideal pick for each person, I just can say it's easier to work with a 3mm aluminum pick and it's easier round shape with it.
You will need:
-Aluminum sheet (3mm works best).
-Sand paper (150->200->300 or more)
Engraving part (optional):
-Paint (I chose black)
P.D: Sorry for the mess.
When finished draw a cross like the one in the picture, then draw a circle, that will give you a basic idea of where do you have to remove material.
P.D: New files can be more difficult to use than old files, I recommend using an old file for this job because it won't get stuck as often as a new one, if you're using a dremel a belt sander or similar then good for you, but take care and check the process periodically because you'll possibly remove too many material and make the pick look uneven.
Trick: Draw the shape of the pick in a block of wood, carve it and make a small hollow space, if you place it there it won't move while you are filing it, I filed it pressing the pick against the table with my fingers, clamps are another option but it's difficult to work with them and usually leave nasty marks on the pick.
Don't give too many inclination to the file, if it doesn't goes down be patient and keep going, filing with a prominent angle isn't as precise as doing it with a slight inclination.
Tip: Get a thin and straight piece of wood and wrap some sandpaper around it, it makes things a lot easier.
Making the edges is a very important and delicate part, to make them place the sandpaper on your hand and start sanding alll the perimeter like you spread butter with it, start pushing with the side, then go progressively tilting the pick and applying less force, do this from one side to another and vice versa to ensure an even edge, you don't want to push it too hard or do it during too much time or you'll end with a round edge, that it's not equal to a progressively rounded edge.
When you've got the edges it's recommendable to sand all the pick again with a fine grit to give it an uniform surface before polishing.
Use the pictures as a reference.
Yes, the red dot on my finger is from too much sanding.
I printed the letters to transfer the design to a piece of tape.
Then, making pressure with a pen I transferred the design to the tape, which I cut with an exact o knife.
I just stick the tape to the pick and dropped some drops of HCl,take into account it's corrosive so if it falls on your hands you'll feel it starts to burn so go and wash them as fast as you can.
Thanks to Kiteman for pointing out aluminum with HCl gives Hydrogen and AlCl, so don't worry about chlorine gas.
I waited like 5 minutes, but 3 are more than enough, you can tell by the deepness of the etch, you can etch a lot of designs that can also improve the grip.
I applied too much oil base paint and after wrinkles appeared I had to remove the paint and start all over again.
TIP:Put soap inside the etch over the paint so the metal filings get stick to the soap and not to the paint, when you are done just wash it with water.
You can now polish the pick, this is optional but highly recommended since a non polished pick can damage the strings.
If you have a polishing machine or a dremel with polishing wax you can do this in seconds, If you don't, well, grab the polisher and the rag and start rubbing, put more emphasis on the edges and the upper parts.
When you find it shiny enough you are done with your pick, enjoy it.