How do I know which terms to group when factoring by grouping?

Question

I've never understood how to consistently group terms in order to factor a polynomial using the grouping method,

$$\text{e.g.} \quad\quad c^2 - 2cd + d^2 - 4b^2.$$

Any advice? How would I know which terms to group in this scenario?

Answer 1

Hint: You may group the first 3 terms as it is a square of a binomial, namely $c^2-2cd+d^2 = (c - d)^2$, and $4b^2 = (2b)^2$.