It is possible to texture over existing textured walls and ceilings as long as your existing texture has not been heavily applied. A heavily applied texture must be sanded before retexturing so that you cannot see it through the new application. Two common texture types are orange peel and knockdown. You can apply new texture right over the top of the existing texture as long as you are using a heavier texture style, such as the random trowel.
Cover the edge of all trim with painter's tape. Tape the edge of your ceiling where it meets the wall if you are not texturing your ceilings. Painter's tape will eliminate hours of cleanup. Use drop cloths to protect the floors in your work area. Drywall compound will wash off with water, but the drop cloths will also save cleanup time.
Fill your 5-gallon bucket half full with drywall compound. Add approximately 1 inch of water to the top of the drywall compound.
Insert the mixer bit into your variable-speed drill and place the bucket of compound between your feet. Hold the bucket tightly while you mix the water and drywall compound together. If you do not hold the bucket, the water and drywall compound will fly out of the bucket when you attempt to mix them. Your drywall compound should be the consistency of a brownie mix -- not too thick, and not too thin.
Scoop drywall compound onto the flat side of your hop board. A hop board is a drywall tool that is used for texturing or as a tray to hold compound when you are using larger drywall trowels that won't fit into a drywall tray.
Dip the flat side of your trowel into the compound. Place the trowel lightly on your wall and drag it. Dip the trowel again, and place the trowel on the wall close to the first area you applied texture and push it across the wall in a different direction. Continue this pattern until you have covered the entire wall.