Move the loveseat to the middle of the room so you can freely move around it. Drape the sheet over it so it hangs evenly. Tuck in the sheet around the edges and back of the cushions.
Attach the button pins to keep the sheet in place. Start with the area around the cushions, reaching into the gap between the loveseat and cushions and screwing the pins in as tightly as possible. Allow just enough give so sitting on the loveseat is possible without causing the sheet to rip or the pins to pop out. Fold and arrange the sheet over the arms neatly and attach pins on the side of the loveseat.
Pin the sheet to the back of the loveseat, again allowing enough give so people can sit without dislodging the pins or ripping the sheet. Look for other spots where the sheet is billowing or loose and push more pins into those places. Once the sheet is tightly fitted, move the loveseat back to the original position. Sit on the loveseat and test it out. Make any needed adjustments.