Correct bandaging is a vital part of first aid. Incorrect application of wound dressings can adversely affect the health of both the injured person and of the person administering first aid.
If you are confronted by a situation in which someone has seriously hurt themselves and is in need of immediate first aid, it is very important that you take a moment to sterilise your hands before you proceed. Your first aid kit should include a sanitising gel, and should also contain a pair of surgical gloves.
Both of these should be applied. Germs on your hands may get into the wound and further harm the injured person, but you are equally at risk. It is vital that you don't come into direct contact with another's blood, in order to avoid contracting any serious illness which is transmitted via bodily fluids.
Once you have ensured both you and the injured party are adequately protected, get on with cleaning the wound and removing any jewellery that may be hindering circulation. Squeeze antibiotic cream from your first aid kit onto the inside of the dressing.
Cover the wound with the dressing. Make sure the material extends well beyond the wounded area so there is space to fix it onto the unharmed skin. Fix the dressing in place by applying elasticised adhesive tape. There should be no movement in the dressing at all, and it should be sealed.