Unscrew the light bulb in your recessed light fixture and set it aside. Reach inside the fixture and unhook the cover trim's metal rods from the grooves inside the fixture. Set the cover trim aside.
Look inside your recessed light fixture for the information sticker, use a flashlight if necessary. This sticker tells you whether or not the fixture is an IC fixture and how many watts the fixture can handle.
Measure the distance from the attic's edge to the recessed light fixture. Transfer this measurement to a piece of insulation and cut it with a utility knife. Push the insulation into the space between the ceiling joists, with one end touching the IC fixture.
Measure from the fixture's other side to the attic's opposite end. Transfer this measurement to another piece of insulation and cut it to length with a utility knife. Stuff this insulation into the space between the ceiling rafters, with one end touching the IC fixture.
Tear smaller chunks of insulation off the left-over pieces and stuff them on either side of the IC fixture.