have a search on here, some good how to's. basically it is sand them with a roughish sandpaper, sand them with finer paper then sand a bit more with even finer paper and finally polish them with a polishing paste, they come up like near new. Best rule is to work in short bursts, do not sand or polish for too long as the lens will heat up and eventually "burn".
Here's one members
how to polish headlights experience
it is very time consuming and labour intensive
