How to show composition of one way function is not such?
You come up with an example: two functions $f,g$ which are one-way functions, but $f \circ g$ is not. To show that the latter is not a one-way function, you come up with an algorithm breaking it.

Here is an example. Choose some genuine one-way function $h$.  Let $f(x) = h(x) 0^{|x|}$, and let $g(x,y) = h(x)$ if $y \neq 0^{|x|}$, and $g(x,y) = 0^{|x|}$ if $y = 0^{|x|}$ (we break the input in such a way that $|x|=|y|$). Note that the composition is just the zero function.