  We present a nonperturbative method for deriving a quasiparticle description
of the low-energy excitations in the t-J model for strongly correlated
electrons. Using the exact diagonalization technique we evaluated exactly the
spectral functions of composite operators which describe an electron or hole
dressed by antiferromagnetic spin fluctuations as expected in the string or
spin bag picture. For hole doping up to $1/8$, use of the composite operators
leads to a drastic simplification of the single particle spectral function: at
half-filling it takes free-particle form, for the doped case it resembles a
system of weakly interacting Fermions corresponding to the doped holes. We
conclude that for all doping levels under study, the elementary electronic
excitations next to the Fermi level are adequately described by the
antiferromagnetic spin fluctuation picture and show that the dressing of the
holes leads to formation of a bound state with d(x^2-y^2) symmetry.
