As I said in a previous post it's not that difficult just a little time consuming and you have to buy the supplies and raw materials. You must have a good core lens to make a new one.
Below is a link to a PDF file that will give you a good idea of whats involved. Certainly not as good way to go for a "production" run of lenses however great for the hobbyist/restorer it will work just fine. Once you have the mold you can cast pretty much all you want. There are numerous other links and video's on how to recast tailight lenses on the web. Google is your friend.
Here is a good example although I am not able to attach the PDF file as it is too large an upload for this site so click on the link below and then click at the top of the page to go to the web page link for the PDF file. Save the PDF file to your computer, open the file and plastic tailight remake 101.
This is a lot better than me trying to explain to you and there are pictures if you save and open the PDF file as directed above. This is a perfect process for those wanting to make an all RED tailight. To make a red/clear would require you to make 2 parts, 1 red and 1 clear and mate the 2 together. No perfect however better than nothing OR you could try dam/separate the 2 colors in the same mold.
A little trial and error goes a long way!
Tail Light Molding with out Equipment.xlsx