First,make sure you have cleaned both surfaces of the watch back and the case surface that meets the back. Carefully line it up and pop the back on. If you have a screw back carefully,after cleaning, line up the threads and rotate the back slowly. To tighten a screw back you may need a 3 prong tool with a handle designed for watches. Of course, if you have o-ring seals check for wear or breaks and replace if defective, because your watch will leak and be worthless. I don't know what kind of watch you have, but be careful and dont hurry or force anything.
Mark G., June 2009
