world coins are easy, they are mostly circle so just pull guides...turn rulers on then take your pointer and click on the rulers at the side of your photo window in PS then hold down mouse button and pull guidlines out and put them at the edge of the top and left of your coin. Get your outline tool (elliptical marquee top left on tool bar in PS, default is square) hold down on the icon in the tool bar till it gives you options and grab the circle outline. start in the corner of your guides where they meet, press down mouse and drag circle out and get it to outline around the coin. Do the same with your other coin...cut (ctrl C) then paste (cntrl V) them together on a new canvas or resize one of the ones you have to a good size 'select - all' (ctrl - A) then 'edit - clear' and paint bucket black on your background layer (select layer in layer window, if it isnt showing its 'windows - layers' to turn it on) and paste them. If you have copied and pasted something it becomes a layer and you can move and resize it 'edit - transform' drag it, or rotate it...resize them to fit, ....on ancients you might have to use the polylasso tool to cut off some extra areas if the coin is jagged our not completely round.
I shoot on black velvet thus if I dont get the cleanest cut...I have a little room for error and its helps to have high resolution shot in macro.




Best thing to do is to try it then ask questions as you go...I read this board a lot and use PS for a living and can help you out as can others I am sure.