Using Lifextender is dead-simple: Install it, run it, then set it up. Specifically, right-click the Lifextender System Tray icon and click Options. If you want the software to work its magic on all your shows, check the Automated Library Scanning box.
Additionally, you can set up rules (by clicking the Rules tab) to include or ignore shows based on certain criteria: title, channel, duration, and so on. In fact, before you let Lifextender loose on all your recorded shows (once it strips the commercials, there's no going back), I recommend setting up a rule so that it scans only one show (maybe one that's not terribly important to you). That way you can make sure it performs to your liking.
The software works swiftly and in the background. When it's done, your shows appear in your Recorded TV library just like before--but without commercials. In my experience, it's fantastic right out of the box: no tweaking of the settings required. It did, however, have trouble with an animated show on Cartoon Network: It always deleted the teaser opening before the credits. Solution: I don't use Lifextender for that show. Other than that, it's been golden.
There are added bonuses to using Lifextender. Not only does it "extend your life" by removing commercials, it produces smaller files that consume less hard-drive space. And those files are easier to fit on DVDs, great if you're into archiving your shows. It's win-win-win.
Lifextender is donationware. If you use it, by all means throw the developer a few bucks to show your appreciation. After all, he extended your life!