The last time I cleaned a horse's sheath, I purchased a very large tube of personal lubricant, such as KY Gel and a few pairs of surgical gloves . Long gloves like the vets use would be best--check with a local vet clinic if you can't find them elsewhere.
Before I started cleaning the horse's sheath I dragged out the water hose connected to the laundry room faucet so I had warm (body temperature) water. Buckets of warm water would work too--but may make it more difficult to get the water to the 'work area'. (Choose a warm day.) I preferred not to use a cleaner with soap in it because there might be a residue after rinsing, despite being thorough. Antiseptic cleaners may be too harsh. There are products made specifically for sheath cleaning , so it is your choice which you use. Once you’ve assembled your cleaning tools and tied your horse, it’s time to go to work.
Put a large dollop of the lubricant into the palm of your gloved hand. You'll have to reapply frequently as you clean the sheath area. I used the whole tube. Then reach up into the sheath and apply it inside. Using your hand, start clearing out the built up residue. Some geldings have what is called a 'bean' or a hardened bit of residue at the end of the penis. You will have to feel for this and clear it away as well.
With KY you don't have to worry as much about rinsing as you would with other products. But do rinse thoroughly. I cupped my hand over the end of the hose and guided it around inside the sheath--your hand will disappear quite a ways up, so don't be surprised. The penis itself sits quite up and back. Some horses will drop for the procedure. The pony I cleaned didn't. He did, however, squeal and cow kick, so be careful.