If you do not have a commercial tourniquet, you can improvise one using a triangular bandage.
Firstly only consider if direct pressure has not worked. You can use a strong pliable material like gauze, material, triangular bandage clothing. To avoid skin damage, make the tourniquet at least 5cm thick. Do not use, wire, string, rope or shoelaces as they will cause damage to the bodies tissues. Do not put them over the wound, fracture or over a joint. They can be placed over clothing or directly to the skin. Do not cover the tourniquet, keep it in view and cover wounds where possible. Mark casually head with a T and mark the time the tourniquet was applied and tell the EMS what you have done and finally do not loosen or remove it.
Learn more about catastrophic bleeding, tourniquets and haemostatic dressings. Video online training at www.protourniquet.co.uk.