Measure the seat of your leather sofa. Measure from one side of a cushion, across the seat, to the furthest side of the last cushion. Also measure from the front edge of the seat to the back of the seat.
Cut a piece of non-skid rug pad to the seat measurements, and place the rug pad on the sofa. The pad will prevent the slipcover from sliding off.
Place the slipcover on the sofa over the rug pad. Use a wooden spatula to tuck the excess cover into the crevices in the sides and back of the sofa.
Secure the slipcover. Roll up a magazine, and use a rubber band to hold it in place. Push the magazine into a crevice. This will prevent the slipcover from sliding out of the crevice. Prepare more magazines, as needed, and distribute them around the sofa. You can use foam noodles instead of magazines if you have some available.