Use this schematic
to build a standard Joule Thief circuit with the added diode.
My joule thief uses twisted network wire passed through a small ferrite core. I use 6 turns of wire. You can find a core from a burned out compact fluorescent bulb. You can see how others have wound the coil and built the Joule Thief, since so many have done so. Just add a diode and LED in series with the charging battery. The LED is useful as a charge indicator.
A high-speed schottky diode would be the most efficient. The 1N4005 was handy at the time and works.