Boy that is easy.

Let's factor this for example

y² - 5y + 6

1.) Set it up as a product of two to where each will hold two terms.

Code:
(     ) (     )
2.) Find the factors that go in the first positions:

Code:
(y     ) (y     )

Remember y*y = y²
3.) Find the factors that go in the last positions:

Code:
(y -  2) (y -  3)

*-2 and -3 are two numbers whose prod. is 6 
and sum is -5
4.) FOIL to double check if you are correct.

First
Outside
Inside
Last

Code:
F + O + I + L

First: 

(y -  2) (y -  3)

(y * y) + O + I + L

Outside:

(y -  2) (y -  3)

(y * y) + (-3 * y) + I + L

Inside:

(y -  2) (y -  3)

(y * y) + (-3 * y) + (-2 * y) + L

Last:

(y -  2)  (y -  3)

(y * y) + (-3 * y) + (-2 * y) + (-3 * -2)
Next we combine like terms:

(y * y) + (-3 * y) + (-2 * y) + (-3 * -2)

y² + -3y + -2y + 6

y² - 5y + 6

It's that easy. Hope this helps.