Roll out the exterior-grade carpet so it covers the deck. Allow the carpet pieces to overlap each other so you have plenty of extra carpet to work with when you cut it to fit. Let the carpet sit outside for one hour so it can acclimate to the temperature. It will expand as it adjusts to the external climate.
Lay the carpet roughly in position so the entire deck is covered. Smooth the carpet so it isn't overlapping and it fits neatly in position. Allow yourself 3extra inches along the edges to make cutting easier.
Push the carpet against the wall of the house using a straight, metal edge. Trim the carpet flush against the house using a utility knife.
Lift the carpet up so you can apply tape to the deck.
Apply heavy-duty double-sided tape around the deck where the carpet edges will go.
Make an "X" shape in the center of each carpeted area with the tape.
Remove the paper off the back of the double-sided tape and press the carpet in position. Align the carpet in its original position as necessary.
Secure the carpet in position using a 75-pound roller that you can rent at a home supply store. The roller presses the carpet and tape together so you get a solid seal.
Cut the remaining edges with the utility knife and straight edge until you have carpet pieces that fit nicely over the deck. Use double-sided tape as necessary around the edges to seal the freshly cut carpet in place. Use the roller to press them down.