I have mares now (and my parents did it when I grew up) but here is some things that I found online (two articles)...I learned something new:
Cleaning a Horse's Sheath
Five things you should know about sheath cleaning for your horse's health and comfort.
By Barb Crabbe, DVM
Fiction: It's a nasty job, but if I clean my horse's sheath frequently, it won't be as bad.
Fact: Your horse's sheath has a population of "friendly" microorganisms that help maintain a healthy balance within. If you clean it too frequently, you'll kill these microorganisms, disrupting this balance--and your horse's sheath is likely to get even dirtier. It's best to clean his sheath every 6 to 12 months.
Fiction: To really clean my horse's sheath, I should use an antibacterial soap, such as Betadine or chlorhexidine.
Fact: To protect those friendly microorganisms, never use antibacterial soap. Use a commercial sheath cleaner (such as Excalibur or Equi-Pro, both available through American Livestock Supply, Inc., 800/356-0700). These products cut through the grease, have a pleasant odor, and make sheath cleaning easier. Warm water also helps cut grease--and your horse will like it better.
Fiction: My horse swishes his tail and sometimes raises a hind leg when I clean his sheath, but he'd never kick me.
Fact: Don't be so sure! Some horses really resent this procedure. They'll not only kick--but they'll do so fast and hard. To be safe, stand well forward, by your horse's shoulder, and reach back to his sheath. If he continues to threaten you, he may need to be sedated by your veterinarian. (Tip: To save time and money, ask your vet to clean your horse's sheath during another routine procedure that involves sedation, such as dentistry.)
Fiction: I can't clean my horse's sheath unless he drops his penis--which he won't do unless he's sedated.
Fact: You can do a very thorough job even when your horse hasn't dropped--simply by reaching up inside. In this case, you'll need to go in up to your elbow, so wear a long disposable glove to avoid getting the greasy, smelly smegma on your arm and sleeve. Ask your vet for an obstetrical glove--it's perfect for the job. You'll also need a bucket of warm water and about 20 heavy-duty paper towels. To clean your horse's sheath, put a generous dollop of sheath cleaner (about 2 to 3 tablespoons) in your hand, along with a wet towel. Reach up into your horse's sheath, and gently work the accumulated grime loose. When the towel gets soiled, grab a new one, and keep working until the towel comes back clean. Clean all the way from his sheath's opening up to the base of his penis. Depending on how dirty the sheath is, you may need to use additional cleaner as you go.
Fiction: Once I've gotten rid of the grime, the job is done.
Fact: Not yet. Once you've removed the smegma, check for and remove the bean-a ball of whitish goo that forms within a small pocket at the tip of your horse's penis. If not removed, a bean can get as large as a walnut and obstruct urine flow, which can cause your horse discomfort and potential harm. If you don't know how to remove the bean, ask your vet to show you.
Finally, rinse thoroughly with clean water and clean towels. Or, if your horse will tolerate it, run the hose up inside his sheath. You'll know you've done a thorough job when the paper towel in your hand comes out as clean as it went in, and the rinse water runs clear.
Barb Crabbe is an equine practitioner based in Portland, Ore.
This article first appeared in the July 2000 issue of Horse & Rider magazine.
How to Clean the Sheath of a Horse
The sheath is a tube of skin that protects the penis. Cleaning a male horse’s sheath is an unpleasant, but necessary job for a horse owner. It should be done at least every 6 months for a gelding and every year for a stallion.
 StepsCut your nails as short possible and/or file the any sharp edges.
put on gloves and then put a tube sock over the gloves.
Ask someone to hold the horse rather the tie him. If you are alone tie the horse safely and securely.
Crouch, or sit on a low stool, as close as possible to your horse's hind legs. The horse will find it hard to kick you there.
Use a sponge or hose to gently wet the sheath and penis, starting from the belly so that the horse isn't surprised.
Lubricate your hand and the sheath well.
Gently insert your fingers into the sheath. The smegma (dried excretions) will usually flake and peel away in your hand. Bring out as much as possible and use a lot of water, use some soap, especially for sheath cleaning, if necessary.
Work your way along the sheath; it is quite deep, until you find the penis. Remove smegma from around the penis.
Check for a bean (a lump of smegma) in the urethra. Insert your little finger into the urethral and feel for a hard lump. If it is there, you will feel it before you get to your first knuckle.
Gently roll it out.
Use a sponge or hose to gently rinse the sheath and penis. If you used soap make sure it is all washed off.
Wash your hands well.
 TipsThe usefulness of sheath cleaning is under debate. Some experts state that it can ‘self-clean’, some state that while necessary for a gelding it’s not for a stallion, but all agree that it is important to be able to handle and examine a horse’s genitals to check for abnormalities.
Rest your free hand gently on the horse's back or loin. This helps you feel your horse's movements so you can move out of the way quickly in case he moves to kick. The touch also helps to relax your horse.
To stop a horse kicking you can ask someone to pick up one of their front legs; however' for long periods this is hard on the horse and human.
Do this in a barn or stable and hope no non-horse people catch you!
Use thin latex gloves rather than rubber washing-up gloves. You’ll be able to feel for any smegma.
If the horse has a very dirty sheath, or large bean, it may have to be cleaned under sedation. Talk to your vet. To avoid this, clean the sheath regularly.
 WarningsSome horses resent their genitals being handled and kick out even when sedated.
Soaps can remove the protective organisms in the sheath. Use only soap specially for sheath cleaning and use it sparingly.
Use only warm water. Don’t test the temperature with your hands as you hands can stand both higher and lower temperatures comfortably, the horses genitals can’t. Aim for body temperature.
The smegma will make your hands smell horrible, so don't do this before an important event.
Don't use any kind of cloth
 Things You'll NeedLatex gloves
A bucket and soft sponge or a hose
Warm water, about body temperature.
Sheath cleaning soap (if necessary). Some ideas are Excalibur (a gel that can be quite cold, so warm it up in your hands prior to application), mild dye- and fragrance-free shampoo, or Ivory soap.
It's not the will to win, but the will to prepare to win that makes the difference.
- Paul "Bear" Bryant (Former college football coach)