How can we find A using finite difference method?

$$ L(u)=u_{xx}+u_{yy}quad 0<x,y<1$$
with homogenous boundary conditions.

I have tried finite difference method,
$$ u_{xx}=frac{u_{i-1,j}-2u{i,j}+u_{i+1,j}}{h^2}$$
$$ u_{yy}=frac{u_{i,j-1}-2u{i,j}+u_{i,j+1}}{h^2}$$
Substituting in our equation, we have
$$ (u_{i-1,j}-2u{i,j}+u_{i+1,j})+(u_{i,j-1}-2u{i,j}+u_{i,j+1})=0$$
after I arranged it and used BC, I got the matrix A.
the steps are provided in link below in more details.
https://www.physik.uzh.ch/local/teaching/SPI301/LV-2013-Help/lvanlsconcepts.chm/lvac_finite_difference_method_for_laplace_eq.html

Thank you.