After finding a rear projection TV on craigslist for free and just barely managing to compress it into a minivan for transport, I began searching for its Fresnel lens. The TV was absolutely enormous, and it was really heavy as well. I eventually found out how to take off the lens and carefully set it aside so it wouldn't get scratched. Other useful things I found in the TV were castor wheels (used in my ball mill), mirrors (CO2 laser, perhaps?), and various circuit boards and lenses that I haven't used yet.
The Fresnel lens itself is quite floppy, and it is also large and unwieldy, which isn't good. The King of Random's video showed how to build a nice frame for the lens that held it rigidly but still allowed for easy rotation to follow the sun. I followed his instructions closely, and the frame turned out to be inexpensive and very helpful. I also used this Instructable to find the lens focal length so I knew where to put objects for best burning performance. My death ray's focal length is about 35". I put a board across the two cross-pieces on the frame so that objects can rest there at a fixed distance from the lens (instead of me holding them in midair). Finally, I scratched each lens side with my fingernail to find out which side had grooves on it. The grooves should face the sun for best performance. In use, I also try to line the lens up so that it is perpendicular to the sun's rays that are hitting it.
Concentrating over a square yard of sunlight into a square inch makes a very bright spot of light, which can be problematic for unprotected eyeballs. I bought some #10 shade cheap-o welding goggles from Harbor Freight and use them every time I play with the death ray. It is impossible to see what is happening otherwise, although my iPad can see details inside the bright spot.
I love using my Fresnel lens. It creates a searing-hot spot of intensely focused sunlight, capable of melting or burning many things. Over the years, I have melted over a quarter's worth of zinc pennies using this Fresnel lens. Yes, melting metal with sunlight! While this lens cannot melt iron or rocks as some videos show, I have melted and compressed HDPE milk jug shreds into a usable billet by placing them in a soup can "oven" under the death ray spot. My Fresnel lens has even burnt all the way through inch-thick wood boards! This death ray cost less than $20 to make and uses freely available energy to do incredibly destructive things, which is exactly why I love it!