If this is a large wound and you need to stop or reduce the amount of bleeding before your veterinarian arrives try wrapping a pressure bandage around it, if the area lends itself to that. This is done by applying gauze over the wound and then wrapping it firmly with vet wrap. Should the bandage become blood soaked do not remove the vet wrap and gauze. Simply apply another layer over the existing pressure bandage.
If the wound is not in an area that can be wrapped use gauze and manually apply pressure. Again, if the gauze become blood soaked do not remove it. Apply additional gauze while continuing to hold pressure on the wound.