Inspect your sofa risers package to see if the parts are all the same height. If not, the taller risers go on the back legs of the couch and the shorter ones go in the front, so that the angle makes it easier to get up from a seated position. Place these appropriate risers next to each leg so that the person assisting you can quickly slide it under.
Put on a pair of gloves to grab the bottom of the sofa without hurting your hands and get a better hold on the furniture. Practice lifting the couch slightly to ensure that you have the ability to lift it enough to clear space for the riser. If not, find another person that will do it.
Squat down close next to one side of the sofa where the arm is and grip the bottom. Tighten your stomach and hold your back straight as you lift with your legs. Meanwhile, have the other person slide a riser under each raised leg.
Repeat the above step to insert the remaining two risers. If necessary, ask your assistant to take a turn in lifting the couch so that you do not do too much lifting.