Explanation:
I am not sure what exactly you mean.
do you mean the complete square of an expression or
term ?
if so, then by multiplying this term by itself, and that means in general, every part is multiplied by every part and the part results are added considering the signs involved.
e.g.
squaring a+b
(a+b)(a+b) = a×a + a×b + b×a + b×b = a² + 2ab + b²
remember that multiplication and addition are commutative (you can flip the right and left sides with each other and still get the same result : a+b = b+a, a×b = b×a).
squaring a-b
(a-b)(a-b) = a×a + a×-b + -b×a + -b×-b = a² - 2ab + b²
remember that
+×- = -×+ = -
-×- = +
+×+ = +
a more complex example ?
squaring a-b+c
(a-b+c)(a-b+c) =
= a×a + a×-b + a×c + -b×a + -b×-b + -b×c + c×a + c×-b + c×c =
= a² - 2ab - 2bc + 2ac + b² + c²