In Peskin and Schroeder Books Chapter 5 subsubection Bound State equation (5.43)
$\mathcal{M}(\uparrow \uparrow \rightarrow B)=\sqrt{2M}\int \frac{d^3k}{(2\pi)^3}\tilde{\psi^\ast}(\mathbf{k})\frac{1}{\sqrt{2m}}\frac{1}{\sqrt{2m}}\mathcal{M}(\uparrow \uparrow \rightarrow \mathbf{k} \uparrow, - \mathbf{k}\uparrow)$
My question : If i would like to calculate the decay width of pion decay, can anybody tell me how to use that equation.
You mixed up eq43 with eq44, you should replace $\left|B\right\rangle$ with $\mathcal{M}(\uparrow\uparrow\rightarrow B)$
But in any case, in the derivation of this expression an important assumption has been made, one which does not hold for pions!
The bound state must be non relativistic to a good approximation. This is true for heavy quark mesons, not for light quark mesons like the pion which is an intrinsically relativistic system (a good way to quickly understand why is to compare the binding energy with the rest energy of the constituents)
