To cast body parts the absolutely best material to use is alginate, it is made from brown algae, and completely harmless, it is also used in food. A bag of fast set alginate costs about £9, and you will need two bags for hands and feet. and
Mix the Alginate with cold water in a clean bowl, make sure to mix while pouring in the water to avoid lumps. The mixed Alginate should have the consistent of a thick porridge when ready, make sure to apply it quickly, since it will set within a few minutes and if the surface isn't completely covered the alginate will go to waste.
Fill a suitable container up with alginate, and cover the body part in alginate, then put the body part to be casted into the alginate, thinking of the posture to be casted in, I wanted the fingers to be straight to work with, and also to avoid air traps when pouring in the plaster.
Wait until the alginate is properly set, about 3 minutes, and then start to move your hand in the cast, the alginate will slowly let go of the hand and you will be able to take it out. Once the hand is out you are left with a negative version of you body part.
Alginate is only good to use as a mould directly after casting, since it will shrink and dry out if left in the air to long, so make sure to be ready to fill it immediately. For this I use the Prestia Casting plaster, If you want it to be durable it is a good idea to use a armature inside, since the fingers might break when demolding, however I did not need this since the finish of the cast is not essential, since I am not using the details, and if the fingers break I can glue them back on.
Once the cat is filled with plaster, leave it to set properly over night. In the morning you can start to remove the Alginate carefully around the fingers, trying to avoid to harm the plaster.