OMG the last time owned a gelding I was 12 and I refused to clean his sheath. My bff had a gelding too so we used to call the vet out and split the barn call to have her do it. But hey.. I was 12. LOL. Now, I'm the one at the barn helping the little kids do it.
The best way for me is to:
get some rubber gloves... sometimes at a pharmacy or surgical supply store you can find the long ones that go up to your elbow, I like those best.
warm water (some people like to use Ivory soap or buy a sheath cleaner at the tack store, you just need to make sure to rinse it really really good.)
a few CLEAN rags
then just go at it.. The more wet it is, the easier it is to remove. If you're lucky he will drop his junk and make it easy for you.

Good Luck!