Some of the answer depends on the age at which you plan to start the horse under saddle. The longer you wait, the more he should know from the ground first, but all horses should lead quietly, keep pace with you, stop when you stop, etc. They should be desensitized to a variety of common things, such as having the lead rope being thrown all over them, especially around the legs, being touch everywhere, holding feet up for cleaning and farrier. If possible get them used to be handled by different people, loading and riding in the trailer, and have several experiences being trailered to new locations without any expectation to perform as possible (could be taking him out to be ponied on a trail, go to a horse show as a non-competitor, etc.)
Before being ridden they should be introduced to the tack (saddle, bridle, bit) and made comfortable with it before proceeding. They should know how to longe at this stage as well.