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The carpet tool has a flat metal runner on the bottom, a handle on the top, and a blade on one edge. Press the tool flat against the carpet and right up against the baseboard on the sharp side. Push it along the wall, applying pressure as you do.  You can use a box cutter for this purpose, but it must have a new, sharp blade. Run it underneath the baseboard to cut the carpet. If it starts getting dull, replace the blade. Also, use a box cutter to cut out spaces for your floor vents. Look along the edge of the strip you cut off. Make sure no strings are connecting the strip to the main carpet, then pull it up and away from the carpet. If you yank up the strip without checking, you might pull fibers from the carpet that cause a run. This tool has a wide, dull blade that lets you press the carpet in place. Use it to wedge the carpet under the baseboard by pressing into the carpet until it settles in place. If you took doors off, put them back in place. Put the door stops back. If you took up floor vents, screw them back in place. Move your furniture and belongings back in place, and you're done!
Run a carpet trimming tool along the edge. Check that you cut cleanly through the carpet before taking the strip off. Press the edge of the carpet under the baseboard with a carpet chisel. Return any hardware you removed.