Fold the armchair cover inside out. Place the cover back on the arm of the chair.
Stand in front of the armchair. Place double-sided masking tape at the top of the armchair cover and run it down the length of the cover to the hem. Cut the tape at the opposite end of the cover. Cut three rows for the front of the cover.
Start the double-sided tape at the top of the cover at the back of the arm chair. Run it down the length of the cover to the hem and cut with the scissors. Place three rows of tape on the back of the cover. Place one row of tape along the top of each arm of the cover.
Fold the cover right side up and place onto the armchair. Press against the cover on the taped areas to secure it to the chair.