Remove your old carpet and padding (underlay). To make carpet removal easier, remove the moldings around the bottom of the walls and the door. Cut the old carpet with a utility knife into strips of 18 to 24 inches in width; whatever is manageable for you. Pull the carpet off of the existing tackless strips (remove these as well), and roll up the carpet and underlay in strips.
Install new tackless strips. A half-inch space should remain between the strips and the wall, with the pins or tacks facing the wall.
Install the new underlay. Place the padding in strips flush with each other, overlapping the tackless strips. Staple the underlay just inside the edge of the tackless strips, and trim the excess. Seal the seams of the padding with duct tape.
Measure and cut your new carpet. Measure the longest part of your floor, and add 6 inches to your measurement. Using this measurement, measure and mark the back of your carpet. Fold the carpet over itself and cut the back side of the carpet with a utility knife. If another piece of carpet is needed, overlap the two pieces, and cut through both to obtain matching edges.
If you need two or more pieces of carpet, put a piece of seaming tape on the floor. Place the tape along the edge of where the two pieces of carpet will meet, adhesive side facing up. Activate the adhesive with a seaming iron. Join the pieces of carpet together over the tape, flush with one another, and seal the seam with a carpet roller (or rolling pin).
Attach the carpet to the tackless strips. Using a knee kicker, which is a carpeting tool, stretch the carpet over the tackless strips, and attach the carpet to the strips.
Attach the carpet on the opposite wall to the tackless strips with a carpet stretcher. Place one end of the carpet stretcher on the end of the already attached carpet, and the other end of the stretcher 6 inches away from the opposite wall. Stretch the carpet over the tackless strips of the far wall.
Attach the rest of the carpet to the tackless strips with the carpet stretcher or knee kicker. Trim the excess carpet with a utility knife or wall trimmer, if you have one. Using a stair tool, tuck the carpet down into the edge of the wall, into the gap between the wall and the tackless strip. Replace moldings.