Wear gloves, long sleeves, eye protection, and a dust mask or respirator
Measure the area and mark the cut line with a marker or chalk
Place the insulation on a flat, stable cutting surface
Use a sharp utility knife for fiberglass, foam board, or mineral wool
Use a straightedge or ruler to guide straight cuts
Cut in several light passes instead of forcing one deep cut
Use a serrated insulation knife or bread knife for thicker batts
Compress the insulation slightly before cutting if needed
Cut slightly oversized for a snug fit, then trim as needed
Keep the blade sharp and replace it when it dulls
Use a saw or hot wire cutter for rigid foam if appropriate
Vacuum or clean up loose fibers and scraps after cutting
Dispose of offcuts according to local regulations