quantum mechanics - How do you build a Lagrangian in particle/nuclear physics? (A specific example)

I know that the terms in the Lagrangian needs to be scalars (with respect to Lorentz symmetry etc.). Also I know that [see C. G. Tully (EPP) p. 85]

in general, for $\psi$ in the fundamental representation and $\vec{\lambda}$ the generators of the group, then

$$\psi^\dagger\vec{\lambda}\psi$$ will behave like a vector in the adjoint representation and can be dotted into a multiplet of fields in the adjoint representation to form a scalar.

So suppose I want to describe the dynamics or interactions of some particles say $\pi,\Sigma^*, \Lambda$. What would be the first steps here to build a Lagrangian? I want to work with $SU(3)$ here.

The $\Lambda$ and $\Sigma^*$ belong to the octet and decuplet respectively.

So the questions are:

1. What would be the first steps in writing this Lagrangian?


2. What shall I take for $\psi$ in this case?