You will fill the crack with a series of small foam wedges.

Use the surform to grind down the sides of the piece so that they are flat and parallel.

You want the wedge to barely fit into its position. You will really have to jam it in with some force. Be careful not to snap it though, as the foam is very brittle.

The wedge should be tall enough so that it extends out both the top and bottom of your repair area. The portion hanging out will be sanded down in a later step.