i got the idea to make this quick short tute after viewing irritants post in pimp an previews, (hope he doesnt mind that i am recreating one of his textures.) i stole most of this method from cochtl hope he doesnt mind also,
well first, make a plane in your front viewport, make it square, and have the corners snapped to grid,
next clone a copy of this plane foreward,
this will be what you model your texture on, the first plane will be what you project the texture onto, you can freeze or hide the first plane if you wish,
now model your texture, you can convert the plane directly into the model by cuting and extruding or beveling, thats what i usualy do, its also fine to add new geometry like lofts or extra boxes as long as you attach it all together (you can do it without attaching but i don't) make surethat the outter edges line up perfectly with your starting plane, its fine to go past the plane, but you must at least go to it.
now light your scene however you would like, i used two target directs one above and slightly angled shining down at 1 multiply, and one aiming up from under with a much lower multiply, the above one had shadows on, also make sure the lights have a large enough radius to light the entire object. now time to project the texture onto the first plane, so select the first plane, and go to your render to texture pannel "0" set your path, then enable projection mapping, click "pick" and select your modeled objects name out of the list,
now click "options" and in the box that pops up click "setup" and enable global supersampling. now go back to your render to texture box, under mapping coordinates tell it to use existing channel "1"
now under output go to add and add a complete map, put in the file name you want for the render, and select a size, i did 512, you can also go back to "add" and add a normal map, if you do this name it, and select a size for it also, now render! take the complete map into photoshop and use it as a starting point for your texture, puting overlays on it as you see fit, and painting in details that you want
well tats all i got, hope its understandable, Writing directions is not my strong point,