Installing wall-to-wall carpet isn't rocket science. By using some specialized tools (available at most tool rental outlets) and being prepared to take your time, it can even be a DIY job.
Step 1 - Remove Old Carpet
Your first step is to get rid of the old carpet. Start by removing the moldings around the floor and taking the door off the entrance so you can get the old carpet up and out easily. Give the old carpet a good vacuuming so you won't be breathing in dust as you pull it up, and then use a utility knife to cut the carpet into strips about 18 to 24 inches wide.
Start at one end and pull the carpet off of the tackless strips and roll it up in sections. Some people feel that you can reuse the existing underlay, but in most cases it will be worn out just like the carpet, so you're better off getting rid of it as well.
Remove the existing tackless strips and make sure the floor is clean and dry. This is a good time to check your subfloor and securely fasten any floorboards that may be loose so they won't squeak under the new carpet (use 1 1/2-inch screws into the underlying floor joists).
Install new tackless strips around the perimeter of the room, but not in front of doorways. Leave a space of about half an inch between the strips and the wall, and be sure the pins or tacks face towards the wall. (They're called tackless strips even though they have two or three rows or very sharp tacks, because using these "tackless strips" means you don't need to "tack" carpet down.) At corners, make sure the tackless strips are butted tightly against each other.
Step 2 - Lay the Underpad
Put the underpad down in strips that overlap the tackless strips. Butt the strips against each other—don't overlap them—then staple the underlay down along the inside edge of the tackless strip. Trim the excess underlay along the inside of the tackless strip and use duct tape to seal the seams.
Step 3 - Climatize the New Carpet
A wall-to-wall carpet will shrink and expand with changes in temperature and air pressure. If you bring a carpet in from outside and install it right away, as the carpet adjusts to the conditions inside your home it may stretch and shrink away from the tackless strips, or may expand toward the walls and wrinkle and buckle in the middle. To prevent this, allow the new carpet to rest, uninstalled, in the room for at least 24 hours. It will adjust to your conditions and remain true to the dimensions you cut it to.
Step 4 - Measure and Cut the Carpet
To install carpet properly, you need to start with a piece that overlaps the edge of the floor by four to six inches. The overlay can then be trimmed so the carpet fits properly. To cut your first section, measure the room at its longest point and add six inches to that measurement. Mark the back of your carpet on both edges with that measurement and join the two marks with a chalk line. Fold the carpet over on itself, and using a straight edge and a sharp utility knife, cut through the backside of your carpet. Be sure to place a piece of scrap board underneath your cut line to protect the underlying carpet.
Step 5 - Set the Seams
If your room is wide enough that you're going to need another piece of carpet, follow the same process with the second piece—measure, mark, and trim. Before you cut, be sure the carpet pile is running the same way in both pieces, and that the carpet piece is large enough to overlap the wall by four to six inches, as well as overlapping the first piece of carpet by four to six inches. Try to lay out your carpet pieces so the seams won't be in noticeable areas even if sometimes that just isn't possible.
Where the carpet pieces will join, overlap the two pieces, and then using a utility knife or a rented seam cutter, cut through both pieces of carpet, ensuring the edges will match exactly. After cutting the carpet, center a piece of seaming tape on the floor underneath where they join, adhesive side up. Use the seaming iron to activate the adhesive (the iron goes on the tape, not on top of the carpet), and then butt the edges together and seal the seam with a rolling pin or a carpet roller.
Step 6 - Attach the Carpet
Use a knee kicker to attach the carpet along one edge. A knee kicker is a solid metal tool about 18 inches long with "teeth" that will grip the carpet on one end, and a heavily padded "butt" on the other. Place the toothed end of the kicker about three inches from the wall and drive your knee forcefully into the padded end of the tool. This will stretch the carpet over the tackless strip where the tacks will grab it and hold it firmly in place.
A carpet stretcher will finish attaching the carpet. A carpet stretcher is similar to knee kicker, but much longer. Put one end of the carpet stretcher against the wall where the carpet is already attached and place the other end about six inches from the far wall. The carpet stretcher also has teeth to grip the carpet, and when you push on the activation lever, it will stretch the carpet over the tackless strip near the far wall.
Work your way around the room stretching the carpet over the tackless strips, and trim the carpet near the wall with a utility knife or a wall trimmer.
Step 7 - Finish Up
Using a stair tool, tuck the carpet down into the gap between the tackless strips and the wall. At the doorway, trim the carpet so the edge is centered under the closed door and install a door edge strip. Finally, cut any vent openings and reinstall the molding on the baseboards.
That's it. Stretch your back, check to see if your knees still work, and then take some time to admire what all your hard work has accomplished.