[Splinters]

I want to make a rosette like on a Martin, the 5-9-5 or something similar. I know how to set up a router and cut a circle but I don’t know the correct bit/bits and pieces to get for the inlays. I assume it’s plastic and not wood. I understand fractional measurements and thousandths of an inch, but when one product is described as 2mm and another is described as 13/64 I get a little confused as to how to match the cutter to the strip.
I have searched and looked for this info but it’s surprisingly hard to find. Someone on here sais they do it all the time, but I don’t know how to send them a personal message.