Oblique asymptotes of a rational function

Homework Statement

To find the oblique asymptotes of a rational function

(i) $f(x)=\frac{P(x)}{Q(x)}=\frac{a_{n}x^{n}+a_{n-1}x^{n-1}+...+a_{0}x^{0}}{b_{m}x^{m}+b_{m-1}x^{m-1}+...+b_{0}x^{0}}$

where $n=m+1$

we exprese it in a form

(ii) $f(x)=ax+b+\frac{R(x)}{Q(x)}$ using long division (my book says). The degree of R is less than the degree of Q.

Q.: How? Does one have to divide (i) and then add? How is the $f(x)=ax+b+\frac{R(x)}{Q(x)}$ obtained?