There seems to be a couple of different variations, with some people putting the lense assy in when the oven is cold and heat it up, while others heat it up empty and turn it off and then putting the assy in.
Obviously you'd want to do it with lower temperature so you wouldn't risk damaging the non metal/glass things.
I've only tried it once with a fog light lense and I failed miserably!
I think the key is do enough research with words like "headlight oven bake", etc... and see which one looks realistic for you to tackle. Also, this goes w/o saying, but be patient!
So i was able to split my headlight but ended up melting two of the brackets. I pre-heated the oven to 350 on bake and set the headlight in glass side down. Good news is I was able to get a heatgun and reform the brackets to where they are supposed to be. I did lose the bracket that attaches under the turn signal but the flange that holds the signal is still in intact. If I did it again I would have let it sit in there for 5mins max be able to split the light without any plastic melting. I did get everything back together and is clean and shiny again.