I realize that many (if not most) of the Instructables audience will be too young to need these, but keep it in mind - sooner or later, you will.
I'm 40 years old and I've worn glasses for nearsightedness most of my life. I'm past being too concerned about looks and contacts are a hassle, so I've stuck with glasses. I have a pair of thin metal framed, small lens glasses that I like a lot. Recently, within the last year or so, I've realized that time of my life has arrived where my arms are getting a bit too short to read comfortably and my really cool new laptop with a 1920x1200 resolution has text that is a bit hard to read when it is sitting on my desk. It's not like I'm completely farsighted (yet), but after a bit, I start to get eyestrain and I read and use the computer a lot.
I'm not ready to get bifocals. I don't really need them and I don't want larger lenses in my glasses (besides, I'm getting old, but I don't want to *look* old). I tried on a low powered pair of reading glasses, over my real glasses, and said, "Wow. That's much better." Of course, wearing two pairs of glasses makes you look a little deranged. I have nifty little spring loaded polarized sunglasses that I bought at Wal-Mart that match my frames so I thought, I'll bet someone makes the same thing in reading glasses. I looked, albeit not very thoroughly, and the best I could find was ones that clip at the top of your glasses and can flip up out of the way. My dad had pair of sunglasses like this back in the 70's and they look a bit dorky.
So, I had just bought a new pair of sunglasses because the lenses in my old ones were scratched and I decided that I would just transplant the lenses from a pair of reading glasses to the sunglass frames. Yes, this would look dorky, too, but at least it would be something that I made and invented myself.
Step 1: Materials Needed
Pair of clip on sunglasses that fit your current eyeglass frames. These can be spring framed, spring loaded, magnetic, whatever. These usually are made for metal framed glasses, although some (particularly the magnetic kinds) are available for plastic.
Dissassembleable reading glasses in the proper diopter. Most stores that sell them have a little reading chart so you can try out the various strengths. If you don't use them already and just think that you might need to (like me), you probably only need +1.00. They go up +3.00 or more. The cheapest ones are only a few bucks, but you probably won't be able to remove the lenses. I bought a metal framed pair (see below) that were rimless and held together by screws. An even better alternative might be to hit thrift stores or yard sales and find a pair. Then you could be more experimental in removing the lenses.
I got smaller reading glass lenses that I needed because my first thought was to make them bifocal like by just mounting the lenses at the bottom of the frames. It turned out that the added distance between the lenses distorted the view a bit, so I wound up centering them.
There is still room for experimentation on bifocal style, but if you're going to mount them centered like I did, then your goal should be to match the size and shape of the reading glasses lenses to the sunglass frames as closely as possible.
Small pliers, eyeglass screwdriver (or a Swiss Army Knife), tape or wire, and glue (I used cyanoacrylate, but there are probably better choices - and I would love to hear of suggestions)