Remove the old gel from the frame for the light fixture. Gel frames are generally two layers of thin powder coated steel or fiberboard. They appear folded over, like a greeting card with one open side – a slot in which to slide the gel. The frame might be secured with gaffer's tape or a staple on the open side. Remove the tape or slice it open, or pull out the staple. Pull the old gel out, and discard it.
Obtain the new sheet of acetate gel for the light. There are a large number of suppliers for stage lighting gels, they come in dozens of colors, but be sure to order acetate made for stage lights as manufacturers construct them to withstand the heat. Regular acetate may melt and cause a fire hazard.
Lay the old gel on the new acetate sheet in a smooth blemish free area. A new gel sheet can often create more than one gel replacement depending on the size of the lights. Mark the size of the old gel on the edges of the new sheet making the marks as small as possible. Don't draw lines or they may show up in the lighting area.
Cut the gel carefully. Slide the new gel into the old frame and re-mount on the light.
Obtain a jewel colored transparent CD case -- commonly found on blank CD's or even sold in packs to house existing CD's and come empty.
Separate the two halves of the jewel case. Take care with the lid, this is the half you will be using. Discard the bottom half with the CD mount on it.
Lay a clothespin on top of the light fixture, with the pincer tip extending just out from the end of the light.
Wrap gaffer's tape around the light, around the bottom scissor of the clothespin, and around again to hold it in place. Wrap several times to make sure it is secure.
Clip the lid of the jewel CD place in the clothespin, using the small protruding edge of the case to catch the lid so that the front of it lays over the front of the light. Shine the light through the lid, and the color of the lid will act as a temporary gel.