Long pants and sleeves
2x4 that is at least twice as long as the width of the insulation
Cutting batts of fiberglass insulation is a frustrating, imprecise process because the batts are so thick and fuzzy. To speed things up and produce a clean, neat cut every time, lay the insulation on top of a scrap piece of plywood with its foil or kraft-paper vapor barrier facing up. Then cut a 2X4 at least 12 in. longer than the width of the insulation.
Place the 2X4 across the batt on the cut line. Compress the insulation by kneeling on one end of the 2X4 and pressing down on the other end with your palm. Then use a sharp utility knife to slice through the batt in a single stroke. Always wear gloves, a long-sleeve shirt, long pants, eye protection and a dust mask when working with fiberglass. And work carefully so the knife doesn't slip and cut your knee.
Fiberglass insulation looks easy to install, but it's effectiveness depends on how snugly it's cut and fit.
To start, avoid packing the insulation too tightly into place. When it's firmly compressed, fiberglass loses some insulating value. Also, don't simply stuff the insulation behind electrical cables and pipes. Instead, take a utility knife and carefully slice halfway through the fiberglass batt. Be careful not to slit the paper or foil vapor barrier on the face of the batt.
And as you install the insulation, fold down the batt slightly to open the slit, then press it over the cable or pipe. The fiberglass will fluff up around the slit and maintain its insulating value.