How to make fake coins?
You take a piece of cardboard,a piece of aluminum foil,a pen,something to cut with,and clear duct tape.First you take a quarter and place it in the center of the cardboard and trace the outside of it and then you cut it out and you have a cicular piece of cardboard and then you take tape and make the tape will be folded and you place it on one side and put it close to the corner but not on it.Then you cut around it and there you go.You can do both sides but you dont have to.Then you take the clear duct tape and you cut a piece and put the fake coin in the middle of that piece of tape.Then you cut around it.Then take a nail or a sharp object and make a hole at the top but not all the way to the top.Then you take a piece of string and put it through the hole and make a knot.And then WAA-LAA!!!You have a FAKE COIN ON A STRING!!!
Considering that actual counterfeit currency is illegal, if you want to just make clearly fake coins, I'd start with card board, or similar, for firmness.
Then cut it out in a circular shape (use a real coin for guidance on size, etc).
After you cut out the coin shape, then color it gray, or silver and write on it using a stencil and or pen / paint to indicate which denomination it is, etc. It won't be a very convincing fake coin, but it could be useful for a prop in a magic trick or similar.