Remove the legs from the packaging and read the instructions that came with the sectional sofa. Some legs, based on style and length, are meant to be assembled in specific slots underneath the sectional.
Move the sectional close to where you want it to be. With the help of another person, flip it on its side. Have the other person hold the sectional piece as you work.
Install the first leg in one of the slots along the elevated side. Sectional legs are typically the screw-in type, so turn the leg clockwise until it has been completely screwed in. Use your wrench to tighten it. Sometimes the leg may be at a slight angle and may have to be aligned in a certain direction.
Move to the next leg along the side that has been lifted off the floor. Install it the same way you did the first leg.
Flip the sectional piece, with the other person, onto its other side so you can easily reach the other leg slots. The key is not to put any weight on the legs that have already been installed. Repeat the procedure until all of the legs are on the sectional.
Test the sectional to make sure that it is level and sitting evenly on the floor. If the legs are not even, adjust them with your wrench after placing the sectional piece on its side.