Some quadratic expressions can be factored as perfect squares. For example, x²+6x+9=(x+3)². However, even if an expression isn't a perfect square, we can turn it into one by adding a constant number. For example, x²+6x+5 isn't a perfect square, but if we add 4 we get (x+3)². This, in essence, is the method of *completing the square*. Created by Sal Khan, CK-12 Foundation.
Check out this link to Math is Fun. Contains visual and algebraic explanation of completely the square. Uses advanced procedural math and vocabulary.
This article contains the algebraic breakdown and explanation of completing the square. It also includes interactive questions that allow you to self-assess as you explore and practice completing the square.
Another video tutorial on completing the square!