Determine which color or colors you want to appear on your backdrop. Take stock of how many lights you have available, since the gels will be attached to the lights. Use a flashlight to shine through the gels to see what color the light actually appears as after it has passed through the gel. It will not always be the exact same color or shade as what you see when you look at the gel itself.
Cut your gel sheets in squares large enough to cover the entire light lens or focusing dish circumference. Test your gel squares by placing them against the surface of your lights. Spot check for any uncovered areas and recut as necessary.
Install your gel squares into the lights by sliding the gel square into the gel holder. If your lighting apparatus does not have gel holders, secure the edges of the gel to the rim of your light with tape.
Activate your lights to see the color they throw.