Find the two arm lugs on the side of the sofa where the arm attaches. The round lugs are often made of steel and protrude from the side of the sofa a couple of inches.
Inspect the arm and find the two brackets on the inside of the arm. The brackets consist of a hole with a slot.
Align the holes of the arm brackets with the lugs on the side of the sofa. Push the arm into the side of the sofa so the lugs insert into the holes in the brackets. Press down from the top of the arm so the lug slides into the slot above the hole, locking the arm in place.