Suppose that $X$ is a finite 2-dimensional CW-complex with free fundamental group and $\phi :K \longrightarrow X$ is a map which induces an isomorphism of fundamental groups, where $K$ is a finite bouquet of circles with the wedge point $a$. Consider the mapping cylinder $M=X\bigcup_{\phi} (K \times \{1\})$. Denote $\pi_n (M_{\phi},K \times \{ 1\} )$ by $\pi_n (\phi )$. Recall that an element of $\pi_n (\phi )$ is represented by a pair of maps $\beta :\mathbb{S}^{n-1}\longrightarrow K$ and $\gamma :\mathbb{D}^n \longrightarrow X$ with $\gamma|_{\mathbb{S}^{n-1}}=\phi \circ \beta$. In the paper ''Finiteness conditions for CW-complexes'' of C.T.C.Wall in Propositon 3.3, Wall has mentioned that since $\pi_2 (\phi)$ is a free $\mathbb{Z}\pi_1 (X)$-module, then we can attach 2-cells to $K$, necessarily with trivial attaching maps, to make $\phi$ a homotopy equivalence.

My question is that:

What is Wall's mean concerning ''necessarily with trivial attaching maps''?

If his mean is that 2-cells are wedged to the wedge point a, then how can I get to this fact?

Thank you for your help.