Obviously this isn't me presenting this, but this is a very clear and concise video on how to do this:
http://www.virtualnerd.com/pre-algebra/factors-fractions-exponents/prime-factorization-greatest-common-factor/greatest-common-factor/greatest-common-factor-monomials
To put it here though:
You have the following:
2w2 and 4w
You want to factor these out to their smallest prime factors
For 2w2:
First the 2, which factors out to 1 * 2.
Then factor the w2, which factors out into w * w.
So the factors for 2w2 are: 1 * 2 * w * w. Don't worry about the 1, so you have 2 * w * w
Now for 4w:
First the 4 factors into 2 * 2.
Then the w factors into w.
So the factors of 4w are 2 * 2 * w.
Now look at what factors they have in common:
2 * w * w
2 * 2 * w
They both have a 2, so you make a note of the 2 (in the video she suggests circling it, I would personally write a 2, and mark through both 2s above so you know that you've already taken those out).
They also both have a w, so you do the same as with the 2 for the w.
Using my way, you would have this:
2 * w * w
2 * 2 * w
2, w
Now that you have a list of all the common factors of the expressions you were given, to get the greatest common factor, you multiply what you have in your list, so in this case it would be 2 * w, or 2w.