Is there anything in particular you don't understand? In most
mods there isn't really all that much to things, or your stock ecig in general.
Battery -> switch -> atomizer -> cart/mouthpiece
Now, in most mods, you make your "battery" out of one or more, wired to a battery connector unlike your stock setup, which has a self contained connector and battery source. You need a switch either inline with the + or - side of your power source to the atomizer so that you aren't constantly applying power to the atomizer and burning it out. And really, that's about all there is to it, unless you are getting more advanced by adding in LEDs, or voltage regulators, etc.