How to clean my bedridden grand after having poop?

It's a Steinmann pin, I think? This is definitely a 2:1 skilled care task - do you have two people looking after your grandmother? You need the two people who know what they're doing to remove the weight (when necessary, not if it isn't), reposition your grandmother, turn her in bed, ensure she is clean and dry, and then replace the weight correctly.

Assuming all that's available, and with thanks and acknowledgements to The Royal Children's Hospital, Melbourne, Australia:

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Ongoing management (of patients in traction)

Maintain skin integrity

Patient’s legs, heels, elbows and buttocks may develop pressure areas due to remaining in the same position and the bandages.
Position a rolled up towel/pillow under the heel to relieve potential pressure.
Encourage the patient to reposition themselves or complete pressure area care four hourly.
Remove the foam stirrup and bandage once per shift, to relieve potential pressure and observe condition patients skin. 
Keep the sheets dry.
Document the condition of skin throughout care in the progress notes and care plan
Ensure that the pressure injury prevention score and plan is assessed and documented. 

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Please tell me you haven't been left to look after your grandmother without adequate equipment, support or at least some basic training and ongoing guidance?

I recommend you ask a nurse on how to clean, and often to check if your grand can not tell you when she goes.

A “cleaning system” or someone to clean her these events occur?
You can gather all supplies in a basin ready to go for you to clean her when necessary but it is imperative someone does it as she is at risk of a pressure ulcer Feces left on skin irritates skin and can cause skin breakdown which is not good.
You can call an agency and hire CNA’s but that will need to be paid for by your mother’s assets or your own money.
Is she eligible for Medicaid? If so she may qualify for a few hours of paid aides depending on your state.