Boy Scout in me is screaming https://twitter.com/sleepisocialist/status/1308592379402780679

Please do not apply a tourniquet unless you are trained. I am Red Cross certified, an Eagle Scout and have taken many first aid courses and I still would not feel comfortable or justified in applying a tourniquet unless there is no chance of advanced medical help

Tourniquets are extremely dangerous and often lead to amputation.
If you are attempting to treat someone with severe bleeding: (next tweet)

W/clean cloth or dressing pad use your palm to firmly apply pressure over the wound. If you have an elastic bandage secure the pad or cloth tightly over the source of bleeding.
If pad becomes soaked with blood, don’t remove it. Apply a new pad and continue applying pressure

If the bandage is on a limb check farther down, every few minutes for: pulse, warmth, feeling and color. If you can’t feel those things of the limb has gone pale, the bandage might need to be loosened.

Once the bleeding stops, hold the pad in place with a sterile bandage(athletic wrap, strips torn from clean clothing etc). Bind pad firmly but not so tightly that it cuts off circulation

You should also, and this is secondary, treat the victim for shock. This is a life threatening condition that can cause organ failure.
If SAFE help the person lie down and raise feet about 12 inches. Keep victims warm and help them to remain calm.

I am not an expert nor do I claim to be. Always call for medical help. Your job is to keep them stable until help arrives.