Now many other people have done something like this before, mine is different in the way that the power supply is replaceable and the case and many of the parts came from an old computer (yay recycling).
As you can see, the main reason why I built this is to power my camera for some time lapse and some long exposures, something the batteries just coulden't do.
Step 1: Parts
Now, parts, I just went to the shack and grabbed all the stuff I needed, you can slowly gather the parts from used audio amps and stuff, but I kinda needed this to work soon so I just bought it. Total cost was near $25 but I had a few of the needed parts already.
You will need the usual 10ohm 10watt sand resistors
a switch (I made the mistake of buying a lit switch, the extra load to light the switch won't turn the PSU on, so get just a regular switch. As an override, I just shorted out the terminals, I'll get a different switch later)
other random stuff, you should probably read through this whole thing before starting your project.