It is a little more prescriptive on the materials, as I know they work, where some substitutes will give you a poor result. If you do this right, and use the materials specified, you will turn your flashlight into a very powerful vapour machine with long battery life.
Step 1: Acquire Your Materials
A dead e-cig battery - from a Janty Ego or a Totally Wicked Tornado. (They are the same, but with different branding.
An aluminium bodied handheld torch from Wilkinson for £3. It needs to be this type because of the way the internals are fixed in place - see pictures. I can't find it on the website, but the bar code is 0320 3545, which may help you find or order it instore.
An 18650 rechargeable battery - This should have a flat top - not a nipple. I recommend this type: http://www.totallywicked-eliquid.co.uk/products/variable-voltage-devices/accessories/evic-2600mah-rechargeable-lithium-ion-product.html
ToolBox Epoxy Bonding Putty from Poundland (See pics)- This is the best I have ever found. Horribly sticky to work with, but when it sets it is unbreakable. Other brands I have tried shatter after a fall from the kitchen table - this really doesn't.
Heavy duty pliers / cutters (must be capable of cutting a zinc-plated masonry nail)
A 25mm zinc-plated masonry nail
An atomiser. There is some flexibility here, but, for reasons I will go into later, I recommend this one: http://www.totallywicked-eliquid.co.uk/products/replacement-coil-systems-2/dc-rcs-series/k1-dual-coil-replacement-coil-system-product.html
Step 2: Disassemble the Torch...
This ease with which this torch can be taken apart and put back together with no ill effects is the reason we are using it. As far as I can know, there is no other torch which is built this way.
If you know different, please let me know in the comments.
Unscrew the switch / battery cover, the torch head and the large spring.
Remove the AAA battery holder from the body.
Turning your attention back to the torch "head," look inside and you will see an aluminium ring resting high up on the internal thread. Beyond this is the underside of the LED circuitboard.
Using a fingernail or the tip of a pen, gently unscrew the aluminium ring until it is completely removed from the head and then turn upside down to shake loose the LED circuit and the clear plastic "lens".
Put all of these parts to one side.
Step 3: Disassemble the Original Battery
Now we're going to take apart the dead battery from your original e-cig so that we can re-use the connector in our new flashlight.
Grasp the body of the battery in one hand and get a good grip of the connector end with a pair of pliers. Work the connector loose with a rocking / rotating motion.
Eventually it will be loose enough to just pull away. Wires inside the battery will pull and snap, and this is where it becomes important that you are using a discharged battery, because if the battery has charge still, there may well be a small fire.
Pull out all of the wiring from the connector including the circuit board and the switch.
Throw away the parts of the battery which aren't the connector.
Looking at the connector head on from the top, you will notice that the central section is insulated from the side wall by a thin rubber seal.
It looks solid but it isn't, so flip the connector over and with a thin screwdriver or a pen, apply gently pressure until the central pillar pops out. Throw this away, but make sure the rubber seal stays where it is - you should probably give it a quick clean with a Q tip.
If the seal pops out along with the central pillar, put it back in.
Step 4: Sticky Stuff
(If you've made the hole a bit too big, don't worry, it just means you may need to tilt the ecig upside down when you change the battery.)
Now find the threaded aluminium ring which used to hold the LED circuit in place inside the torch head.
Slice off a thin strip of Poundland's Toolbox Epoxy Putty and apply thinly around one of the flat (not threaded) surfaces of the aluminium ring.
As mentioned, this stuff is Horrible to work with and will stick to everything, but it is the absolute best that I've ever used. Again, if you've found something different, tell me in the comments.
Press the transparent plastic circle into this. scrape of any excess from the inside and edges, then go and make a cup of tea while it dries.
Step 5: Bits, Pieces and More Sticky Stuff...
The aluminium ring / transparent plastic assembly should have dried by now. Think of the plastic side as the underside and the other side as the top.
Push the nail through the bottom of the assembly an then place the connector on top of this with the head of the nail poking through the top.
Use putty again to fill the gap between the connector and the ring, but leave a gap of about 5mm - 10mm in one place around the edge.
Let the putty set while you go and go out for a bike ride for a few hours.
On the interior of the battery holder which you removed at the beginning, there is an array of springs. These can be unscrewed and removed without tools. Unscrew and remove one.
Compress the spring and place it into the gap between the connector body and the aluminium ring. Its own tension will keep in in place and there is now a direct connection between the internal wall of the connector and the aluminium ring, which will in turn be in contact with the body of the torch and the negative terminal of the battery.
In addition, this spring acts as a fuse. If there's a short anywhere in the system, the spring will quickly heat up and collapse in on itself in a puff of foul-smelling smoke. This will save your battery / face / life. Do not be tempted to substitute anything more solid for the spring. It is important.
Step 6: Nail Clipping
Mark on the nail where it comes through the seal.
Remove the nail from its current location
With your heavy duty pliers / cutters snip off the excess.
Replace the nail.
I would have dearly loved to have shown photos of this process, but my camera has been replaced by a potato. Nonetheless, here is a photo showing the lengths of the nail before and after snipping.
Step 7: Final Assembly
You may recall that back at the beginning, I recommended a particular atomiser / tank system. This is for two reasons.
The first is that as long as I have used it, it has not leaked. Excess leakage into the cigarette means that the fuse spring will glow red hot and collapse in on itself, necessitating a replacement.
The second is that the Central connector, which will be in contact with the nail is spring loaded. This means that if your nail is slightly the wrong length, it will not be pushed inwards damaging the atomiser and shorting it out.
The third is that it's a dual coil replaceable head system, meaning lots of vapour and easy to maintain
Screw the atomiser / tank onto the connector. If you apply gentle pressure to the bottom of the nail, you may feel the action of the spring in the tank system. This is fine.
*****If you are using an atomiser / tank system without a spring and you notice that the nail protrudes further when the atomiser is attached, this means the nail is too long and your atomiser may become damaged. You need to either trim the nail shorter or make sure that when you assemble the electronic cigarette, you do NOT screw down the atomiser as far as it will go.*****
Find the (now hollow) head of the torch and fit the aluminium ring into the thread so that the atomiser pokes out of the top. Use the atomiser to turn the entire assembly until it will turn no more. When this point is reached, the atomiser will detach. Don't worry about it for now.
Screw the torch central tube into the head and insert the 18650 battery. Screw in the battery cover / switch/ end bit.
The battery will be pressed firmly against the switch and the underside of the connector assembly and providing tension to prevent the assembly turning when the atomiser is screwed on again.
Screw the atomiser on again.
Press the switch on the bottom of the flashlight to activate your new electronic cigarette.
The battery should last around two days of very heavy ( pretty much constant) vaping before it needs to be charged again, and put out enough power to produce a frankly ridiculous volume of vapour.