  We establish a systematic construction of the on-shell amplitude/operator
basis for Chiral Perturbation Theory (ChPT) in $D=4$ spacetime dimensions and
with an arbitrary number of flavors $N_f$. For kinematic factors, we employ
spinor-helicity variables to construct the soft blocks, which are local
amplitudes satisfying the Adler's zero condition, as well as to take into
account the reduction in the kinematic basis due to the Gram determinant, which
arises at $O(p^{10})$ when the number of multiplicity $N$ in an amplitude
becomes large: $N>D$. For flavor factors, we include group-theoretic relations
at small $N_f$, $N_f\le N$, which decreases the flavor basis. The result is
obtained by adapting the Young tensor method of constructing the operator basis
for generic effective field theories to the case of non-linearly realized
symmetries. Working in the massless quark limit, we present purely mesonic
operators for both even- and odd-parity at $O(p^6)$ and $O(p^8)$ for $N=6$ and
arbitrary $N_f$, and establish a direct correspondence between the amplitude
basis and the operator basis. Furthermore, the redundancy due to the Gram
determinant is studied at $O(p^{10})$ for $N=6, 8$ and 10.
