Hi this is Dr. Gabby,
It depends what you have on hand.
Vaseline or Mineral oil will soften it up.
Or, take some dish washing soap and rub that into the feces.
If she will let you, and you have a sprayer on the shower or kitchen sink, the easiest thing to do is to put her in the sink or tub and spray her hind end until the feces come off. That is what I would do. Hold her by the scruff of the neck the way a mommy cat carries her babies.
You can also put a wash cloth or towel over her head. Cats think their body is hidden if you cover their head.
If you have clippers, and not scissors, you can usually get the clipper blade under the dried feces. Then if there is skin irritation from the clippers close to the skin you can apply over the counter hydrocortisone cream every 12 hours.
I have an 18 year old pet with the same problem. I use the sprayer or the clipper technique.
Does that help? Do you have any additional questions?