To get started prepare the damaged area by removing all old texture from it. Next, sand it smooth, if any of the damaged texture remains, your new texture may not stick. Prime the damaged area
Match the texture of the area that you are patching. To get a close match, use a paint additive instead of a premixed spray-on texture. This will let you to have more control over the size and concentration of the aggregate used to get the popcorn effect. Aggregate comes in fine-, medium- or coarse-grade aggregate. Mix a small batch and paint it on a spare piece of plywood or drywall. Gradually add aggregate until you’ve matched the existing ceiling
Protect the area around your targeted area. Cover them well, because this texture technique is quite messy. Have water and rags or sponges available to quickly clean any messes that may spread beyond your protected area.
Apply the texture. You can use a roller for most areas. If the repair is close to a wall, you can use a brush or edger to get into the areas the roller can’t reach. Apply the first pass until it overlaps the existing texture.
Randomize the effect. Without dipping your roller or brush back into the texture, go back over the area in several directions, applying from the wet areas to the dry areas. This minimizes any pattern that may develop from the first application. Don’t overwork the area. You should stop after a minute or two of working.
Allow the area to dry. If the repaired area has less texture than the existing ceiling, you can add a second coat of texture. If it’s acceptable, paint the repair using the same paint used on the rest of the ceiling.
Remember to always wear goggles or safety glasses and any other appropriate safety equipment.