Normally when trimming the hair around your dogs bum, I would take a short hygiene blade and run it upwards towards your dogs tail to clear any hair around their bum. The chances are you won't have clippers so you can use scissors but ensure you have someone to hold and also ensure you do not point your scissors towards the dogs bum and keep them flat at all times.
You should scissor and just clear any hair where poo could get caught or trapped. The person holding needs to keep the dog very still and if you sense your dog will move, then move the scissors anyway.
Its annoying but not the end of the world if your dog has a hairy bum. If they keep getting poo trapped, just shampoo the area and rinse it out. This is better than potentially cutting your dogs bum.
Below are images to show you how to place your scissors when trimming around your dogs bum. Place your scissors flat near your dogs bum, DO NOT point them towards their bum - if they suddenly move you could injure them if you have wrong scissor placement.
Place your scissors like this.
DO NOT place your scissors like this.