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The best way to tackle this is to put her in a bath or basin with a hand shower. With warm water (careful not hot!). Irrigate away the poo with the shower. Use gloves because you may have to massage it a bit to break it down. If you don’t have a hand shower stand her in the basin with the water up to the level of the faeces and again massage it to break it down. Regularly refresh the water. Don’t use scissors! I see many cut bottoms on dogs because of scissor accidents. Dry the area with a towel once it is totally clean.
Once the faeces are removed you should inspect the skin around the anus. It will probably be red and sore. If it isn’t too bad it may not require any medication but usually I prescribe a topical cortisone/antibiotic cream. Alternatively any soothing cream designed for skin rash will help.
Of course you need to work out why this happened and try to prevent it happening again. Shi Tzus have long coats that can tangle with the stool as it exits. Once one stool gets stuck the rest follow suit. It may also be that she has had diarrhoea. Check up on that. Professional groomers will be able to clip the coat shorter around the rear end to make it easier for her stools to be passed.
Finally, if you feel the skin around the rear end looks infected and very sore, get her to a vet. She may need antibiotics.
Good luck, Peter