First, you need some copper foil -- stained glass stores have copper foil with an adhesive backing. Depending on how you want to finish the soldering, you'll want to get the foil with the color your final solder color will be on the back.
You can get a wide enough roll to either cover both sides of your slides together, or just enough to cover the edges of one or the other.
Tape up all sides of both pieces. If you have the smaller size foil tape, tape the seam between the 2 pieces.
You'll also need a soldering iron, flux, and, if you're making this as jewelry, lead-free solder. Soldering is really easy. All you want to do is make the solder flow and cover the foil. It's hard to explain exactly HOW to solder, but here's a video I found. http://www.youtube.com/watch?v=hzvk6eMCyfU
You apply flux (to clean the metal) then heat the solder up very close to the surface. I'd practice to make sure you can solder the slides without cracking them (easy to do if your soldering iron is hot, and you heat up the glass too much).
There are patinas to change the silver color of the solder to either copper or black, depending on what effect you're going for.
Hope that helped!