The third ingredient: external legs

Two ingredients down — the vertex and the propagators. The third is the edges of the diagram: the lines that run off to the detector, carrying the real particles that actually come in and go out.

Each external leg contributes the wavefunction of its particle. For fermions it is a spinor; for photons, a polarization vector. The role depends on whether the particle is incoming or outgoing, and on whether it is matter or antimatter:

An incoming fermion brings $u(p)$, an outgoing fermion $\bar{u}(p)$. For antifermions the two swap places: an incoming antifermion brings $\bar{v}(p)$, an outgoing one $v(p)$. An incoming photon contributes a polarization $\varepsilon^{\mu}(p)$, an outgoing photon its conjugate $\varepsilon^{*\mu}(p)$.

That is the last piece. With external legs in hand, every line in a diagram has a factor — and the dictionary is complete.