You have two options: DIY or buy. I know you said DIY, but it will most likely cost you more in effort, parts, tools and wasted food while you learn.
If you are after the end result, then you should probably buy something like Traeger Smoker or similar. It burns compressed wood pallets and has electronics to monitor temperature and keep it within required thresholds. Considering that something yummy will take you 6 hours upwards, automation helps! I have the smallest model which works great. Longest smoke I've done on it is 12 hours, worked fine. Just chuck the fuel in, turn it on and come back when it's done. Pretty much that easy. From the smoker point of view anyway, you'll still need to mop or spray or whatever else the recipe requires.
If you are dead set on building one, search for Ugly Drum Smoker (UDS) instructions. Your main challange will be seal and temperature control. Temperature control in low and slow cooks with charcoal fuel is not as easy as it looks on YouTube. UDS is pretty much the defecto DIY project if you want to build one. A friend of mine built one and it's also great. But! Temperature control requires a lot more attention. It takes a lot longer to get it going, get it to the the required temperature and stabilise on that temperature/maintain that temperature for hours, it's not as easy or predictable as people make it out to be. You will need to watch it. At the end, my friend ended up building an embedded computer (connected to his smoker) connected to a small PC fan and a thermometer; which starts the fan when the temperature drops...
Which brings us to the next big expense. GOOD food thermometers. When you cook meat in the smoker or meat in general, you cook to temperature, not time. You will need a fast thermometer because last thing you want to do is open your smoker - rapid heat loss, so you want to keep that to a minimum which means a very fast meat probe ($100) . Most people who are in to this stuff also own a 'remote' thermometer. Something with 2 probes - food and internal unit temperature, then send it to receiver unit so you don't have to run back all the time to check on it.
Slight tangent, if you have an old charcoal style Webber then you already have a smoker, most people just don't know how to use it right... Watch BBQ Pit Boys (
http://www.youtube.com/show/bbqpitboys) and you'll see how to use it like a smoker rather than a BBQ. And if you don't, you could still turn something with a heat source into a smoker - watch this
https://www.youtube.com/watch?v=CJjvKtfAJXc (or build one of this, about $50 worth of parts, most cost is in the aquarium pump on eBay).
Finally, better forums to ask this on are:
http://www.aussiebbq.info/forum http://www.aussiepitmasters.com.auHope it helps.