This paper does not say how the author finds apparent horizons, but [68, page 135] cites a preprint of this as treating the apparent-horizon equation as a 2-point (ODE) boundary value problem: Eardley uses a ‘beads on a string’ technique to solve the set of simultaneous equations, i.e., imagining the curve to be defined as a bead on each ray of constant angle. He solves for the positions on each ray at which the relation is satisfied everywhere.