Remove the cushions from the sofa.
Drape a king-size top sheet over the back of the sofa and tuck it between the back and the seat. Tuck it into crevices where the arms meet the back as well.
Tuck one end of a new sheet into the crevice between the seat and the back of the sofa. Let the other end drape over the seat and the front of the sofa to the floor. Tuck excess fabric into the crevice.
Cut one sheet in half lengthwise with sewing scissors. Drape one half over each arm of the sofa. Tuck the excess fabric into crevices so the sheets are tightly pulled around the arms and the sides of the frame beneath the arms.
Replace the cushions and lay a new sheet over the cushions. Tuck the end into the crevice between the cushions and the back of the sofa. Tuck the other end under the front of the cushions.