First, lets use the long solution.
Our example is (a+b)²
now you have to write it as (a+b) (a+b)
now (a+b) (a+b) equals a²
then (a+b) (a+b) equals 2ab
then(a+b) (a+b) equals b² 
now as a final result, the answer is a² + 2ab + b²

Other long solution    a²      +   2ab      +   b²   
                                  (a)(a)      2(a)(b)     (b)(b) 

now the short cut, (a)(a) is a², 2(a)(b) is 2ab, (b)(b) is b². Then our answer is a² + 2ab + b²

Tip: Square the first term
       Twice the product of the first and second term
       Square the second term