Which DVDs Have the Optimizer?
A whole bunch. Any DVD or Blu-Ray disc with the THX logo has the THX optimizer embedded. You can also check this search-able list on the THX website.
Do I need Anything Else?
Yep. For the best calibration, you'll want to grab a pair of Blue Filter Optimizer Glasses. You can order them from the THX site and get them sent to you for less than a fiver ($1.99 + $2.50 shipping). But you can use the optimizer without the glasses, too.
When Should I Do It?
If you're prepping for a party, do the calibration the day before, at the same time of day. The idea is to simulate the lighting conditions you'll be watching in—including whether blinds will be drawn and lamps will be left on. If you're generally calibrating your TV, try to find a happy average of your TV-watching conditions. A moderately-lit room without any window glare should work well.
Anything Else to Know Before I Start?
Yeah, turn off your TV's smart settings. If you have any kind of "auto" or "dynamic" smart picture settings on your TV, turn them off or they'll mess with your manual calibration adjustments.
So How Do I Do This Thing?]
It's easy. Run the THX Optimizer from the DVD menu, then follow the on-screen prompts. Or, just follow this guy's lead: