get silicone rubber and make a mold. Make the mother mold out of plaster to save money, time and effort. There are on-line tutorials which can help you.
To teach mold making in one post is not really do-able. Go to the library and get some books or on-line help to visually see how to make a mold. I've included a picture of my Webley mold in production to help out.
A few things to keep in mind is to completely seal up the original so rubber doesn't flow in the wrong places. Avoid undercuts. Make sure the rubber is thick enough so it wont tear and there make a pour spout...
Just a start - perhaps somebody else is better with words?
