Another day and suddenly you watch a road accident where no other help could arrive soon more than your knowledge and there's no hospital near. The victim considerably bleeds thus without an appropriate assistance, perhaps it would be fatal. Let's review or recall some methods!
The best advice is to download a first aid manual and carry it in a mobile device like a smartphone. If there's no time to read it, the steps to rationalize would be the following:
Pressure to stop the blood loss
It's more or less obvious that the objective is to stop the blood flow to the exterior.
Something like a bandage would be ideal
Every vehicle should carry a first aid kit, but if this is not the case clothes are something with a similar functionality. Different options, but of course better those that don't create eventual infections or sticking problems.
Lay the victim, trying to moderate the blood flow
When an extremity is in an upper position than the heart, it receives less blood flow.
A normal temperature of the body avoids complications
Then, with a warm clothing to cover the victim should be enough.
The quantity of blood loss should be less than 30% of the total
For reference, a person with 75 kilograms should have more or less 5 liters, so about 1.5 liters of loss cause very serious issues. Looking or palpating might provide a reference of the loss.
The last option is to cut the blood with a tourniquet
This method focuses to cut the flow from an access point, with great risks to the extremity. To avoid these risks, the tourniquet must be loosened every 15 or 20 minutes. Less time if possible is better.
These protocols don't try to be a substitute of a proper qualification and documentation, please be responsible and check for other sources of information as well.