There are several of these around the net, but most people use an altoids container, a 5v voltage regulator and a 9v battery. I built one of these and it sucked. Because 9v bats only hold around 450mah at best, it charged the phone about 5%, which rendered it useless. It also uses a female USB plug, which means you have to carry around a USB cord with you, which is extra hassle. I wanted to create something that used common easy to find parts, and was cheap to build. This whole project costs around $4 to build, and it charges the phone anywhere from 25-40% depending on the type of batteries you use.
Step 1: Parts List
1x Battery box - $1.99 (I saw it at another Radio Shack for $2.29, but my local one had it for 1.99)
4x AA batteries - $.50 (I went to the dollar store and bought this 8 pack for $1)
Cell phone cord $1.50 (I found one cheap on Amazon, but make sure you get the one that fits your phone)
1A diode (these are very cheap if you buy them in bulk - probably in the 10 cent range).
Soldering iron / solder
Heat shrink tubing, or you could use electrical tape I suppose
Cut the black and red wires coming from the battery box leaving about 1.5 inches left.
Take your diode and cut it leaving about 1/2 - 3/4 inch left on each side. (You'll notice that this is a 3A diode and I mentioned before that you'll need a 1A diode. I didn't have a 1A diode and didn't want to buy one, so I used this 3A one. But 1A is all you need. Wall chargers allow 1A.)
* Heat shrink tubing is cheap, and if you're a tinkerer, grab yourself a box of them, you'll use them all the time. But if you don't have any, all this can be accomplished with electrical tape, it just won't look as nice.
Solder the black to black 1st, then wait till if fully cools before slipping the tube over the connection. Once on, heat it to shrink it, and move to the red. This is where the diode has to go. Make sure the diode is soldered in the right direction, otherwise current cannot pass beyond this point. The stripe on the diode represents the cathode side (negative) and it leads away from incoming current. So this side (see the picture) will be on the opposite side of the batteries, or the side that is closest to the phone plug. The solder the red wire coming from the battery box to the side of the diode that doesn't have the stripe (the anode).
Wait until fully cooled and then slide the larger tube that you had already slipped on before you soldered the diode on. Heat to shrink.
Put in your batteries, close the lid and charge away.
* This box takes 4 AA batteries and if you know anything about batteries, you know that non-rechargeable batteries carry a voltage of around 1.5 volts. Most rechargeable AA batteries carry a voltage around 1.2-1.3 volts. We need at least 5 volts here to charge a phone. Your USB cord, whether plugged into a wall jack or computer, will allow around 5v to pass because that is the minimum voltage needed to charge a 3.5-3.7v phone battery. So 4 rechargeable batteries would give you about 4.8v, and even though technically this would momentarily charge a 3.7v battery, especially if it's a dead battery, it will take a long time to do so and would probably only give you about a 5% charge. 4 regular batteries gives you 6v and will not only charge the phone more, but also much more quickly. Since these are cheap at the $ store, this is what I used. With the cheap ones I get about 25-30% charge from a dead battery. If I put in Duracell bats, it will give be around 40-50% charge, but this is an expensive way to go.
And that's it. Remember to turn the switch on!! =) Good luck.