Not really a lot to say about this - trace around the uke body on the back panel, cut about 1/8" to 1/4" outside the line, and glue the sucker on.
Well, OK; there is more to it than that :-). Gluing on the back is very important because it solidifies the relationship between the centerline of the neck and that of the body. If you don't get it right, then either the strings won't line up with the fret board nicely, or the bridge will have to be installed off-center in the body. So put down the beer, straighten up and pay attention to what you're doing!
I've made a dedicated jig for this, but all you really need is a nice flat surface with a straight line drawn on it. Weight and/or clamp till the glue dries.
Next: Cutting the sound hole
If you find any of this helpful I'd certainly appreciate a "tip" :-)