Heres a simple way to cast pewter, heres my way of casting, theres more info out there online aswell. This is an ancient technique that allows the casting of basic shapes without the cost of expensive mould materials.
Cuttlefish wash up on the shores all the time, all thats left when found on the beach is a white oval shape with a soft side and a hard bony side.
What you need:
A few cuttlefish( can be found at the beach or pet stores have them for Parrots!)
Small off cut of hardboard.
Leadfree pewter.
Blowtorch or small camping stove.
Sanpaper.
files.
Hacksaw.

Step 1: Make the Mould.

So get hold of a nice dry cuttlefish bone, if its damp it wont take a mould and the hot pewter will spit. Cut off one end of the cuttlefish bone with a hacksaw, so that you have a square end.
There are two ways to make the impression in the mould, you can carve away at the soft side with small files and nails or you can use a simple piece of jewellery you have around and push this into the soft side of the mould, so that when its removed it leaves an impression. (see photos)
Flatten the soft side of the bone with some sandpaper wrapped around a bock of wood. Then make the impression about half inch or so down from the top of the flat end of the cuttlefish, then with a file make the pouring sprue from the top of the mould. This is where the molten pewter will be poured in and when finished it gets cut off.
Take a small piece of hardboard that will fit over the back of the cuttlefish, this will become the back of the mould, rub this board lightly over the soft side of the bone so that it sits flat.
In the photo of the cuttlefish with the round shape, Im making a ring with a flat top so this mould has a sprue to pour into and a hole at the base for the metal to flow into another mould for the ring top. (but thats another story!)

Step 2: Clamp the Mould

File a groove in the top of the smooth side of the hardboard to match the one in the cuttlefish. Now place the smooth side of the hardboard onto the softside of the cuttlefish so that both slots line up, this is where the molten pewter will flow into the mould.
I use a couple of metal clamps to hold the mould together, but soft wire can be used aswell, make sure the two surfaces of the mould fit well together or liquid pewter will escape during pouring.

Step 3: Melt the Pewter

Support the mould upright with the pouring slot at the top, make sure its on a heatproof surface. A couple of blocks of wood or bricks will stop it from falling over.
A few warnings at this point, wear leather gloves and a pair of goggles or a face mask to protect against burns, if the mould isnt quite dry it may spit back hot metal. Also any gaps around the joint in the mould will allow hot pewter to run out, so theres an obvious fire risk, also protect your skin from burns and you might like to have a fire extinguisher handy.
Melt a small amount of pewter in your spoon or whatever you want to use thats heatproof and pour slowly into the slot in the mould, which will fill quite quickly, stop when pewter is seen at the top of the slot.
Let things cool for about ten minutes or so and then carefully open up the mould and pick out the piece with a small file. You will get marks on the surface of the pewter from the cuttlefish but these can be polished out. You should be able to get a few casts out of each mould if you repair the surface after each casting. Which is fine because the cuttlefish are cheap (or free)
Cut off the sprue with a hacksaw and then file off any marks and polish with a dremel or polishing wheel.

Step 4: A Few Pewter Pieces

Heres some photos of pieces of pewter jewellery that can be made, Ive made these with a combination of cuttlefish moulds and hardboard. Have fun with it, watch out for hot metal and let me know if you have any problems with the casting.