I collect unique buttons or findings that I can make a mould from. Here is a button I had of a Christmas package that I thought would make cute earrings or charms. To make a mould from it I start by running my scrap clay through the pasta machine at the thickest setting and stacking 2 or 3 layers -- depending on how deep the impression will be.
I spray my clay with a mould release agent -- I have a spray bottle of water that I like to use. Take your item and firmly press it into the clay as evenly as possible until you get a nice deep impression. When you take your piece out, inspect your mould to see if it looks good. If it doesn't, you can smoosh your clay and then start over and try again.
When the mould looks good, the excess clay is trimmed away so it looks neat and tidy. After baking I allow my mould to cool and it is then ready to use.
Here is a photo of some of the many moulds I have made over the years: