The first thing you need to do is gather all the parts you need. For this instructable, you will need 1 Eveready Industrial Flashlight, found at Home Depot for $3.97. You will also need 1 6V flashlight bulb from Radioshack; model KPR112. This is about $1.50. Finally, you will need 3 123 batteries. These can be found on surefire.com for about 2 dollars a piece. Total cost for this project is slightly over $10.50.
Step 2: How to modify the flashlight.
First, you need to unscrew the end cap end the lens cap. Inside you will notice two spines running the length of the light and another two with a strip of metal between them. All you need to do is remove one of the spines that is not next to the metal. To do this I used a chisel from a generic X-ACTO type kit. After this, I taped the 3 123 batteries together so they would be easier to insert and remove. Then I inserted the batteries. It is a very tight fit. It will require some pushing. A tight it fit is better because it prevents the batteries from rattling and from excessive shaking. Replace the standard bulb with the KPR112. Screw both ends together and you should have a working tactical style flashlight.
If you look at the first image below, at 135 degrees there is no spine