Clean and prep the floor. Cut lengths of tackless strip to fit each wall around the perimeter of the room. Secure them with the pre-driven nails into the subfloor, keeping the strip out from the wall about 2/3 the thickness of the carpet. Make sure the tackless strips join together at the corners and the pointed pins in each strip are facing the wall.
Note: If you are replacing carpet, pry up the old carpet from the tackless strip. The padding and tackless strips can be reused if they are still in good shape.
Measure the room to cut the padding. Be sure to measure from the longest points of the room, but measure from inside the tackless strip edge. Cut the padding to fit using a straightedge and utility knife. Lay out the padding waffle side facing up, and staple it along its edge every 6". The padding should not overlap; it should be butted up against each other to form a clean seam. Use a utility knife to trim the excess padding that might be covering the tackless strips, and cover each butted seam with duct tape.
Measure between walls along the largest parts of the room. Transfer the measurements to the carpet, adding 4-6" in all directions. Cut the carpet on the back edge with a carpet knife and a straightedge. A chalk line can be helpful for transferring longer measurements.
Position the cut carpet in the room, fitting the factory cut edge against the first wall, followed by the other sides. Use a carpet knife to cut around any odd corners of protruding walls.
Use a knee kicker to force the edges of the carpet onto the tackless strip. Hook the carpet to the tackless strip starting in one of the corners. Dig the teeth of the knee kicker into the carpet about 1" from the wall. Swiftly kick the cushioned end of the knee kicker with your knee to hook the carpet to the tackless strip.
Use a power stretcher to firmly stretch and secure the carpet onto the tackless strip on each side of the room. Once one corner is hooked with the knee kicker, use the power stretcher to stretch the carpet to the opposite wall. Put the base of the power stretcher at the wall that was just hooked with the knee kicker.
Tip: Use a piece of scrap carpet or a padded 2x4 if necessary to pad the wall.
Dig the teeth of the power stretcher into the carpet about 6" from the opposite wall. Press down the lever and lock it into place, stretching the carpet and attaching it to the tackless strips. Use the power stretcher to hook the other corner opposite of the first corner hooked, following the same procedure. Continue with both the knee kicker and the power stretcher around the entire room until the carpet is tight and secure.