Although one of the dirtier aspects of horse ownership, cleaning a horse stall isn't that difficult if performed on a regular basis—and doing so can provide you with a much happier, healthier animal.
First, it's best to remove the horse from the stall before you begin because the cleaning process may spook the animal (not to mention that it is often much easier to clean a stall without your horse in the way).
Depending on the bedding material used, to clean the stall you will need a wheelbarrow (or other appropriate item like a cart) to haul the debris out and to bring new bedding material in along with a sturdy pitchfork, stall pick, shovel or rake (or a combination of all four).
To start cleaning, remove any visible debris littering the stall then move around the walls, pulling the bedding away from the edges to check for moisture, debris, fallen feed, etc. Remove any wet or dirty stall bedding you may find. If there is a natural floor like clay or earth, use a shovel to clean up any damp ground. Also check along the walls of the stall for any signs of insects, rodents, etc. so those issues may be addressed.
To clean the central area of the stall, again use your pitchfork or rake to sift through the bedding, checking for damp areas caused by urination or water spills as well as for buried feces or spilled feedstuff. Also look around for cobwebs, bats and bird nests and do an inspection of the feed and water buckets in the stall (clean and refill as needed).
Once the stall is cleaned of all debris, using your wheelbarrow dump the soiled waste outside of the stall or barn—it may take more than one trip—then add fresh bedding as appropriate, spreading it out uniformly over the floor of the stall. Make sure it is spread evenly and that any hay is not left matted or clumped together in large pieces that will trap moisture.
When you are done, do a last minute look around to be sure that the area around the stall door and the floor is clean and safe and that everything is functioning properly. Now you can bring your horse back in to its fresh, clean stall.
It isn't hard to clean horse's stall but if while cleaning you find there is more "dirty" material than clean (and you're cleaning at least once every week), it is best to remove all existing bedding from the stall, coat any wet spots on the stall floor with a horse-safe ammonia removal and/or disinfectant product, let it air dry for a day, then replace the stall bedding with new, clean material.