Final answer:
A player must clean and dress their wound properly before re-entering the field.
Step-by-step explanation:
When a player is sent off the field due to an open wound or blood on their clothes, they need to take certain steps before re-entering the field. First, they should clean and dress the wound properly to stop the bleeding and prevent infection. Then, they should ensure that the wound has healed and there is no risk of reopening or further bleeding. Once they have taken these precautions, they can safely return to the field.