For cutting thick leather I often clamp the leather between 2 bits of hardboard or plywood and use a hole saw in an electric drill. Or: cut out as best you can with a sharp knife/scissors. Remember to allow plenty of size so that the finished cup washer is OK. Remember you can always trim the outside down if too deep. I would normally add about 1" to the piston diameter.
Once I have a leather disc, I treat it with a shoe leather stretching liquid (iso-propyl alcohol) which makes it pliable. Or soak a day or two in neatsfoot oil. Then fasten disc to end of piston using the original fixing parts (bolt, screww, washer etc). Now get a jubilee clip that is a loose fit on the piston body. Feed the leather disc into the clip and tighten it a bit to begin forming a cup. Keep tightening at intervals until the jubilee clip is tight around the piston body. You may have to move it a bit now and then to keep the disc central. Once at the 'clip tight' stage it is best to leave it soaking in neatsfoot overnight. When ready, you may need to trim the outer edge a bit but remember that when compressed, the outer edge will move back to give a flat bottomed cup so do not overdo it. Try the piston and seal for fit. If too tight, carefully reduce outer diameter of the cup with something like 240 grit (cleaning after!) or even a few strokes of a file or put piston assembly in a lathe and trim a bit of leather off the cup. Aim for a good sliding fit.
Breech seal can be a bit more fiddly to cut. I usually use a set of wad cutters ... about £6 a set from tool stalls. I cannot remember the exact measurement for a Falcon but many rifles are 12mm outer diam and 8mm inner. If your leather is not thick enough, just add a card or steel shim under the washer. I have also cut discs from plastic tubing (e.g. home brew tubing) and it works well.
Happy shooting.
Cheers, Phil