Rock tumblers make the surface of rocks smooth. The combination of sand and water used in the tumbler polishes and smooths the rocks. Rock tumblers are either mechanically driven or manual. You can make a manually operated tumbler out of items around your home. Once you've seen the outcome, a polished/smooth rock, you might want to pursue the hobby further and purchase an electronic rock tumbler.
Fill the container you chose to use halfway with sand. Any type of sand will do, but it is best to start out with coarse sand, as it speeds up the smoothing process.
Fill two-thirds of the container with water.
Mix the water and sand together. You'll use this mixture, called "grit," to smooth and polish the rocks.
Add the rocks to the grit mixture and cover the container with the lid. If you don't have the original lid, use any lid of a similar size and duct tape it down so it isn't loose. Use about two handfuls of rocks if the container is the size a 326gr coffee tin.
Shake the tumbler in all directions frequently over the next several days.
Depending on the roughness of the rocks, manual tumbling can take several days or up to a month.
Do not use a glass container; the rocks may break it when you shake it. Do not overfill the container with rocks. The rocks will not be able to tumble enough, and this will prolong the smoothing process.