Carpeting can develop "ripples" over time due to stretching of the carpet itself, compressed padding due to wear, tack strip failure or unravelling at the edges. Re-stretching the carpet can extend the life of it, improve the appearance and improve safety. Both manual and power equipment kits for restretching carpet are available at tool rental companies at reasonable daily rates. You should be able to restretch an average room in a day at most. Padding and tack strips can be purchased at flooring dealers and often at home improvement stores.
Determine what has caused the carpet to ripple. If the edges of the carpet have unravelled, you can't restretch it since there is nothing for the tack strips to grip.
Remove the furniture.
Use a knee kicker extended to the dimensions of the room to restretch the carpet over the existing tack strips if they seem to be gripping and the ripples are not extensive. The knee kicker is what it sounds like and will stretch the carpet forward to catch on the tack strip as you hit it with your knee. A power stretcher is an alternative to consider.
Carefully pull the perimeter of the carpeting loose if the padding or the tack strips need to be replaced or if the rippling is extensive.
Replace the padding if needed. Pry off the old tack strips and discard. Use a hammer and chisel to cut the replacement tack strips to length, and nail them in place about ½ inch from the wall.
Lay the carpet back down, and use a knee kicker first to engage the carpet over the tack strips. Use a power stretcher, if desired, to work it until tight.
Use a carpet stair tool to fit the carpet between the tack strip and down along the baseboards.
Use only the same thickness of padding that was on the floor if replacing it. Your carpet may not fit if you use a greater thickness.
Do not over stretch or you will risk tearing seams or even the carpet itself. Knee kickers are hard on knees and any flooring project should not be attempted by people with back problems. Be careful when handling tack strips as the barbs are very sharp.