I blogged this for a neophyte on Energetic forum.
If you’re going to use an existing lantern, then you may have a lot of modifications, since a lantern usually has several cells in a battery pack, which gives you a long battery life. If you reduce the number of cells, the battery life will be considerably reduced (remember that the battery voltage cannot be as much or more than the LED voltage). joule Thiefs are only about 50 to 60 percent efficient, so you will lose up to half the power in the circuit. You could instead use a more efficient circuit such as my SJT.
For 12 regular LEDs, you could just buy a couple of the 9 LED flashlights for about 2 dollars apiece, disassemble them and remove the circuit board, and remove a few of the LEDs from each. Then build a more powerful JT circuit to give enough juice to run all 12 at their rated brightness. You will have to do some serious work to get enough power out a simple JT circuit to put out up to 240 milliamps to the 12 LEDs.
If you’re going to start from scratch and not use something already assembled, you have many more options available. One option is to just put four JT circuits, each with three LEDs, built into the same case. A good idea is to use more powerful 10mm LEDs, as Quantsuff did in his project. Go here and scroll down to Sample Retrofits.
Back to experimenting…