In the research paper Factoring integers with the number field sieve by : J.P. Buhler, H.W. Lenstra, Jr., and Carl Pomerance
https://openaccess.leidenuniv.nl/bitstream/handle/1887/2149/346?sequence=1
under 5. The algebraic sieve : they have used norm mapping $N: K \rightarrow Q$ , where $K$ is a number field and $Q$ is the rational field.
Later they are claiming that norm of elements of the form $a+b\alpha$ can be computed by substituting $a, b$ in the homogeneous polynomial $(-Y)^{d}f(-X/Y)$ that is, if $a, b \in Z$ then
$N(a+b\alpha) = a^{d} -c_{d-1}a^{d-1}b+...+(-1)^{d}c_0b^{d}$
where $f = X^{d} +c_{d-1}X^{d-1}+...+c_0$
I'm not getting how norm is equal to this $N(a+b\alpha) = (-Y)^{d}f(-X/Y)$. I'm new to this field. Please explain it as easy as possible. Thanks in advance.