Let's say you have an equation like
x^2 + 6x
to complete the square you must make sure that the second term (6x in this case) contains the first one. the rest is just formula application.
a^2 + ba
(a + b/2)^2 - b^2
so in our case
a^2 = x^2
a = x
b = 6
b/2 = 3
(x+3)^2 - 9
I hope that helps.!