Place a piece of rug grip on top of the furniture cushions (where you sit) before you place the slipcover over the furniture. The rug grip will adhere to the fabric of the slipcover and help keep it from shifting as you sit on the furniture.
Place the slipcover over the furniture. Adjust the slipcover to center it properly and begin tucking the slipcover into the crevices of the furniture. As you tuck, make sure the edges of the slipcover are properly straight and at the correct length along the back, sides and front edges of the furniture.
Use a wooden spoon or spatula to tuck the slipcover snugly into the crevices along the sides and back of the seat cushions. When you use a spoon or spatula, you can reach more deeply down into the cracks of the cushions and secure the slipcover fabric more tightly into these crevices.
Insert PVC pipes, wooden dowels or magazines rolled up and held with rubber bands into the crevices along the back and sides of the seat cushions. Push these items down as far as possible into the cracks and they will hold the slipcovers in place.
Check the slipcovers every day to make sure they are staying in place. Preventative tucking and straightening daily will help to prevent the slipcovers from coming untucked.