You need a dedicated microwave oven and a pyrex measuring cup. Do not use your food microwave. Do not do this in the house either.
You also need an open pour mold. They are pretty cheap on Ebay, lurecraft etc. They come in a silicone, aluminum, urethane, plaster of Paris, etc.
You can also make your own mold from a bait you like. Open pour molds will have a flat side so craws work well.
Melt old plastic in micro and stir every 15-30 seconds as not to burn it. When it becomes a liquid with no clumps in it, pour in the mold and let cool.
These are the basics. There are other ways to heat plastic such as an electric pot or hot plate.
There are also many safety precautions to take also as 350* plastic can do ALOT of damage..