It's not that difficult. Any textbook about QFT starts with analyzing free fields, and for these free fields everything can be solved analytically. It turns out that for free fields there is a particle interpretation in the sense of a complete set of states with definite particle number, where the particle number can take any integer (including 0) value. Given a one-particle basis (for massive particles usually defined by the Lorentz boosts of a particle at rest with definite spin-##z## component or for massless particles by the Lorentz boosts of the quantum with momentum in ##z## direction and definite helicity ##\pm s##), the complete Hilbert space is spanned by the occupation number eigenstates for these single-particle states, and depending on the integer or half-integer spin you necessarily must quantize the fields as bosons or fermions, respectively (spin-statistics theorem following from microcausality and boundedness of the Hamiltonian). In this way you get a definite particle (or massless quantum) interpretation of QFT. This is all pretty straight forward and just a bit of more or less complicated math of the representation theory of the proper orthochronous Lorentz group and some thought about the discrete factors of the rest of the Lorentz group (parity, time reversal, and charge conjugation).
Now the trouble starts when you switch on interactions. Here, the particle interpretation breaks generally down, and (except in some academic toy cases in low space-time dimensions) you can't solve the problem to construct the Hilbert space out of the Lagrangian/Hamiltonian and symmetries as in the free case. Usually, what's done is to restrict oneself to the socalled S-matrix that describes the transition probabilities from an asymptotically free initial to another asymptotically free final state, i.e., you start usually with two free particles and ask what's the probability (per unit time and unit volume) that these particles scatter to some final state with again asymptotic free particles, and you use perturbation theory to evaluate these transition probabilities. This evaluation is tremendously simplified by introducing the ingenious notation in terms of Feynman diagrams, which even suggest to interpret them as the "microscopic mechanism" of this scattering process. The important point is that one cannot take this intuition too far. The internal lines of Feynman diagrams, which are just a clever notation for the expressions of perturbation theory to get the S-matrix elements, are called "virtual particles", but that's a misnomer since even in the sense of perturbation theory, there's no way to interpret these mathematical expressions, the socalled free time-ordered propagator (which in vacuum QFT is identical with the Feynman propgator), as particles in any way. A more close to physics interpretation is indeed as in classical field theory (i.e., electrodynamics) to see it as a solution of the field equations describing the interactions leading to the scattering process whose S-matrix element you want to calculate. A proper physical interpretation in terms of particles are only given by the asymptotic free Fock states of the free theory, and these are represented exclusively by the external legs of S-matrix Feynman diagrams, and stand mathematically for certain coefficients in the plane-wave solutions of the corresponding free field equations.
Another complication is that as soon as you have massless quanta in the theory (e.g., photons in QED), the above given picture of asymptotic free particles is too naive, and one usually has to resum an infinite number of Feynman diagrams to get useful results (e.g., for bremsstrahlung in QED even at the tree level), i.e., you have to dress the "naive" plane-wave solutions of the free particles to describe the long-range interactions described by the massless fields/quanta. In other words the true "asymptotic free electron" in QED with it's own electromagnetic field around it is not fully described by the free solutions of the Dirac field, but in this picture is always surrounded by a "cloud of soft photons" or better said "coherent em. fields".
In short: The "particle interpretation" of relativistic QFT is much more involved than in non-relativistic QT or even suggested by the apparently "intuitive pictures" of Feynman diagrams.