For a pressure dressing
When someone is bleeding, applying pressure will usually stop it. A very clean, wadded up T-shirt comes in handy for this. First, you can place it between your hand and the wound to help protect yourself and the bleeder from each other’s germs. (Use nonporous gloves too if you have them.) For jagged wounds, the T-shirt will also allow for deeper pressure. For large wounds, it will allow for pressure across a bigger surface area. Here’s the best trick: If the bleeding stops with the pressure but starts back when you release it, you can make a pressure dressing so you don’t have to keep holding the T-shirt on the wound yourself. Just tape the shirt down firmly onto the wound. The tape acts as your hand, continually pressing the shirt’s bulk onto the injured area. This should keep the bleeding stopped. Check more of my genius first aid tricks for when help is NOT on the way here.
For a tourniquet
If a leg, arm, hand, or foot doesn’t stop badly bleeding even with pressure, it’s time for a tourniquet. No makeshift solution works as well as a commercial tourniquet. However, if stopping the bleeding is a matter of life or death, and all you have is a T-shirt, it’s worth a try. Tear a strip that’s four to six inches wide and two to three feet long. Fold it until it’s about two inches wide. Wrap it above the wound, and tighten until the bleeding stops. A sturdy stick can help with this: Tie the ends of the T-shirt to the ends of the stick, and twist the stick like a steering wheel to tighten. Get expert help as soon as possible because you can’t keep a tourniquet on too long without causing severe tissue damage.