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Fibrin comes from an inactive precursor protein called fibrinogen (fibrin maker). Fibrinogen is dissolved in the blood.
The fibrin filaments are made when fibrinogen is broken into fibrin molecules, which can link together into long strands.
The 'trick' of clotting is to make sure that all of this only happens when there is an injury, and that it only happens at the site of the injury!
So what about an injury will activate the process?
The first step is actually simple -- at the moment of an injury, the smooth muscles in the blood vessel spasm and the vessel constricts for just an instant. When that happens, the platelets in the blood brush up against the injured vessel wall, and stick to the collagen in the exposed subendothelial layer. They do this by attaching to one side of a protein in the blood called von Willebrand factor. The other side of this protein binds to collagen.
Here's an injury with the von Willebrand factor attaching to the collagen on exposed subendothelial cells, and the platelets attaching to the von Willebrand factor. It acts as a link connecting the platelets to the injury.