Well, if you want to make a two-part press mould that you can only use once, and that won't work for whole figures, as it's too complicated to handle, here's how you could do it.
- you need two blocks of plasticine (or one block cut in two, depending on the size),
- make sure they're not too soft (otherwise, put them in the freezer to get just a bit harder).
- make sure the surface of both blocks is perfectly flat.
- press the item that you want to copy into one block (easiest is to follow the original mouldline)
- add some locators (beads are good for that) that you also press in the block (when you have made both halves of the pressmould,these will make sure it's aligned properly).
- press the second block onto the first
- gently remove the second block again
- gently remove the item you want to copy, but leave the locators in place
- put the two mouldhalves in the freezer until they are rockhard
- mix some putty (greenstuff or other)
- give it roughly the shape and size of the final item and put it in one mouldhalf
- add the second half of the mould and press hard
- do NOT separate the mouldparts but rather leave it to rest for a day or so until your putty has cured.
- peel off the (now softened again) mouldparts and remove your copy
- clean the mouldlines and excess putty
Only do this for your own use and don't go selling copies on ebay (besides, the process is too time consuming for that since you have to start over from the beginning each time).