A figure 8 follow through is the exact same knot as a figure 8 bight, but the knots are tied using different techniques. The follow-through technique is used when you need to tie into (or around) an existing object. The figure 8 follow through is used, almost exclusively, to tie into a harness.
Start by tying a figure 8 skeleton knot leaving a tail that is a little less than an arm's-length long. Then take the tail and thread it back through the knot, retracing the rope's original route. Dress the knot and leave a tail that is at least fist-width (3-4") long.
Wish you were rigging?
You could be.