Double Tonguing consists of a forward stroke of the tongue followed by a backward stroke. On a single note a flutists says "Tu." For Double tonguing there are a few ways. Some ways are easier than others. The first way is to say "Du-Ga-Du-Ga," So the forward stroke of the tongue is a "Du" and the backward stroke is a "Ga." Another way of doing it is to say "Tuh-Kuh-Tuh-Kuh." Same as above the "Tuh" is the forward stroke, and the "Kuh" is the backward stroke. Note that the Tuh-Kuh is a much stronger and harder type of tonguing and usually isn't as nice as the du-ga.
Now that you know that, practice each part on its own. So if you use the first method you would practice "Du-Du-Du-Du-Du." Once you get that down, you would practice "Ga-Ga-Ga-Ga-Ga." Once you get conformable playing both of those separate you would join them together to form "Du-GA-Du-GA," Or "Tuh-Kuh-Tuh-Kuh," depending on which one is conformable to you.
If you are still having trouble speeding it up, you can also try: "doo-goo-doo-goo" or "da-ga-da-ga." The problem with da-ga is that it is two different syllabus and in the long run, this will slow you down.