Clean out all of the grain. Make sure you thoroughly clean out your silo so it's not infested with bugs or any other small animals. If you don't clean it out well now, you're only increasing the chances you'll have to give it a good cleaning later.
Install a ladder on both the inside and outside of the silo. Most silos should already have ladders, but make sure yours does before undertaking this process.
Seal all of the holes on your silo. A good tool for this is liquid concrete, which will serve to waterproof your silo. Pay close attention to the base of your silo. Years and weather may have caused it not to be as flush with the ground as it once was, and if that's the case this will be a major leaking issue.
Run some water from a hose at the top of the silo. As referenced above, the smallest tower silos are usually around 30 feet. If one cubic foot equals 7.48 gallons of water, then 30 cubic feet would be 224.4 gallons of water. It's going to take a while.