If you are going to just cast little pieces like shields/weapons/model parts, the easiest and cheapest way with decent results would be Greenstuff + Bluestuff.
Bluestuff is reusable so you dont lose anything if you do a mistake while moulding etc.
Check this video to get a feel about how it works.
https://www.youtube.com/watch?v=mVZLXLaidjQPewter/Resin/Dental Stone Casting is more advanced and honestly not worth the trouble, unless you are doing to make a lot of high quality pieces/whole miniatures or terrain.