You need to find a website with a countdown creator. Hopefully someone else can give you some links to them, but one that I know of is http://www.**************/ (I just found another one, too http://www.magicalkingdoms.com/timers/)
After you create your banner, they will give you the image link to the banner. You then must go to your control panel "User CP" on the top of the page and then click on Edit Signature. (You can also just click here: http://www.disboards.com/profile.php?do=editsignature)
Then you want to put the code in the signature box. The code should look like this: