Step 1: Materials Needed
A wall transformer,or power supply; mine is 3.7 volts. The output of your charger will vary depending on the voltage rating of the battery you want to charge.
A small, incandescent lamp; this will use up the remaining power in the battery in discharge mode to prepare it for charging. I used a Christmas bulb, which works fine for 3.7 volts. You'd need a bigger one for a higher voltage.
SPST switch; for those who don't know, that stands for Single Pole Single Throw; it's the kind with three terminals on the bottom. I used a push button one, but any kind except momentary would work.
Wire; any kind that will carry the required voltage and current - which, in this case, isn't very much. You'll need about a foot, at most.
Clips or plug; You'll need something to connect the battery to the charger; my batteries terminated in bare wires, so I used clips (binder clips, as it happens, because I didn't have alligator clips). If your battery terminates in a plug, you could use a corresponding socket.
Something to hold everything, such as a small box. An altoids tin would work if you insulated it.
You'll also need something to cut the necessary holes in the box, and perhaps a soldering gun, although you could manage quite well without one.