Everything is pretty easy to source and not too difficult to assemble!
First of all, I am really impressed by the easy2led housings. They are really well manufactured and considering the options that you have with this housing, it really is a steal. They have an inside diameter of 25mm (one inch) that makes it easy to fit small 3Up LED stars or the new, powerful XML-Stars. The housing is divided into one smaller (around 11mm) and one bigger (around 21mm) chamber that give you the possibility to either use a better and bigger optic or to use a more advanced and bigger driver unit. Of course you also have a back cap and a front lens that screw into place.
On the following picture, you can see the LED Star fixed with two small screws. As this light is intended for rough off-road use, you definitely don´t want loose parts rattling around. Another possibility would be to use a bit of thermal adhesive under the star and then to keep everything in place with the optic and the front lens.
The LED star that is used here is a single CREE XML U2 copper pcb, which results in a very good heat dissipation. Those stars are available here, for instance.
The next picture shows the back cap with a waterproof (IP65) cable gland. The most important point is that the cable gland is used as a strain relief. These small lights tend to get tossed all over the place, so you need a cable gland that secures the cable really well.
The driver used in this particular light is a very simple and inexpensive linear driver with around 2800mA current (claimed, probably a bit less in reality), that you can find on ebay or DealExtreme.
In addition, I recommend using some silicone wire which is very flexible and heat-resistant. The wires should have a decent diameter to handle the 3A current.
Here you can see the generous space for the 20mm optic that is held into place with a dedicated holder. These holders are snap-in types that make sure the optic is levelled inside the housing, resulting in a cleaner spot. I needed to cut a bit of the holder so that it would fit over the screws that hold the LED into place.
Finally, you need some batteries to power this little beast. As a single XML LED will draw around 3V, you can go the easy way and use a combination of four Mignon-Type batteries (AA size). These batteries will give a combined voltage of 4.8V, easily enough to drive the LED (please read the information regarding battery types further down). In addition, chargers a pretty cheap or probably already available from other electronic devices lying around. You can use a very basic holder like the one in the following picture and just store it in your backpack, or you can get a more rugged one with a hard case and possibly a switch. In either case, these holders are very cheap and not too heavy, which comes in handy if you prefer to mount everything to your helmet.
IMPORTANT: I really would not advise anyone to use "normal" Mignon (AA) batteries for high-power LED applications. The light will draw up to 3A of current from the batteries, which is far more than normal batteries can take. The result may be blown batteries, short circuits and a destroyed light, or worse. This is why I would suggest to use ENELOOP batteries, that are superior to any other Mignon (AA) batteries that I know of, and can repeatedly deliver very high currents. They are not much more expensive than normal batteries and are available almost everywhere. You will need a fresh set of batteries to get the most out of the light anyway, so do yourself a favour and get ENELOOPs.
If you want to mount the light on your handlebar, I can really recommend the HOPE mount. It is pretty affordable, lightweight and has the perfect size for the housing used here. It should be available at your local supplier or at all major online stores. Here it is mounted on the older light with 3 CREE XPG LEDs.
Once you have everything connected you are ready to go. This light is small, light and relatively cheap, nevertheless it is easily powerful enough for all trail riding duties.
If you need more information regarding DIY bike lights, I would always recommend these sites:
mtbr.com - DIY lights
IBC - Elektronik rund ums Bike