Measure the width between the two joists that house the recessed light fixture. Transfer this measurement to two boards that are the same width as your joists. Joists may be between 2-by-8-inch to 2-by-12-inch boards.
Cut the board with a circular saw so you have two short boards that will fit between the joists. Measure 3 inches from the recessed light on both sides and make a mark on the joists.
Line up the boards with the marks on the joists, keeping a 3-inch gap between the recessed light and the boards. Hammer 2-inch nails through the joists and into the short boards to hold them in place.
Measure the length of the gap behind and in front of the short boards that you need to fill with insulation. Transfer this measurement to a strip of fiberglass insulation. Hold a level across the insulation. Cut the insulation with a utility knife, using the level as a straight edge. Stuff the insulation into the gap between the joists.