Depending on the severity several things can be done. Direct pressure applied to the wound will slow bleeding. Elevating the wound above the heart will help. If the blood comes in a squirting/pulsing manner it may be necessary to apply a constricting band between the wound and the heart. It is not recommended to apply an actual tourniquet unless it is a life and death issue. These methods can be learned via a Red Cross First Aid Class among other life saving techniques.