Unroll and lay the carpet out for at least one hour to let it expand if you are cutting a strip to fit in a specific spot. According to Lowe's, this should be done when temperatures are between 55 and 95 degrees Fahrenheit and relative humidity levels are between 10 and 65 percent.
Hold a straightedge firm against the spot where you want to cut to the carpet. If possible, have a second person hold the straightedge while you cut. Straightedges are sold specifically for this purpose, but you can use a yardstick or a long, straight piece of wood.
Run a sharp utility knife along the straightedge to cut the strip of carpet. This should be done over concrete, plywood or another hard surface that won't be damaged from the utility knife.