Take off the weather stripping and any screws that are on the plastic. Throw them in the oven for about 10-15min at 250*F. Keep a close eye on them and watch them should anything happen, but shouldnt. Put the headlights on a cookie sheet or something so they dont touch the rack of the oven or the sides of the oven. Use some gloves and start prying a bit at the clips holding the headlight together. Once you got the clips, just start pulling apart the headlight. Its pretty easy. Was difficult for me the first time but now ive done 3 different headlights it takes about 3min after taking them out of the oven. For a beginner, i would recommend keeping the oven on while you pull the headlight apart so you can quickly throw them back in the oven and make the glue soft again.
You can also use some black RTV silicon sealant to put them back together. I needed two tubes on my last set of headlights (one for each headlight). Also, there's a couple of how-to's in the how-to section detailing the cracking open process but it's pretty simple after you get through the first one.