So firstly you need to get some white plastacine from your local art store. Break a piece off and warm it up in your hands. Roll it out on a flat surface so its approx 1" deep. Press the item you want to use as your "master" carefully into the plastacine, making sure you press it down flat and trying not to disturb the plastacine around the sides of the object. If using a more detailed piece you will have to be careful removing the item from the plastacine.
You can see in the picture below I have used a small metal frame and two buttons.
Mix and pour the resin as instructed on the box, pour very carefully as the impressions are not very deep. You will see below I have a little overflow but don't worry as that can be trimmed off later.
This photo shows how you can easily replicate a button even if you only have one left in your craft stash.
Again fill with resin as normal.
The resin sets hard just like when using shop bought moulds, although it does take longer than normal.
In the photo below I have pulled the items from the plastacine and you can see that they have overflowed and attached themselves together. Don't Worry.
In the photo below you can see that when you pull the set items from the plastacine they don't not come away cleanly. just pull as much off with your fingers and then using warm soapy water and a scrubbing brush give them a good wash.
The overflow excess can be cut away with a pair of old scissors and you are left with your completed items ready to use. all you have to do now is reroll your plastacine and pop it in a sealed bag and it will stay soft ready for using again.