First, paint your lightning as three continuous zig-zag lines. Neatness is not a prerequisite here as you can see. Try to at least keep the different lines obvious enough that you can follow them with the eye at this point.
Next, choose a line to follow. In this case I chose the second one from the tip. Paint sharp edges of the background color at the first place where it intersects another line to give the impression that it is going over that line.
At the next intersection, paint edges along the other line to give the impression that your chosen line is going under that line. As you go, clean up any fuzzy edges with more of the background color.
Continue alternating over and under. You don't need to stick with the same line all the way through as long as each line alternates. If you make a mistake, just paint the lightning bolt color over the edges and then draw the correct edges.
Sharpen corners and edges as you go, continuing the process out to the points of each line.
That's it. Pretty simple, eh? This would probably work for tribal-style tattoos as well.
|fig 1. basic lightning shapes|
|fig 2. first edging over|
|fig 3. second edging, under|
|fig 4. alternate over/under|
|fig 5. continue and clean up edges|