How can you cut a hydrocolloid bandage to fit a wound?

With scissors! A good way to get a dressing to fit a wound is to gently press the paper side of the dressing to the wound. This will imprint the dimensions of the wound on the dressing. Cut it so that there is at least 2 cm from all edges of the wound in order to ensure adequate coverage, adequate adhesion, and minimal wast.
Medical supervision. Should be required for use of hydrocolloid dressing & thus dr or health care prodessional treating underlying problem should demonstrate and advise. Is used for wounds, pressure ulcers, acne, superficial burns etc. But advice on it's use is, in my opinion, beyond the scope healthtap.