well, save it on to your comp, as a JPEG file. Then go to your profile, ((by clicking on either User CP or Private Messages at the top of the page, then clicking on Edit Profile at the top of the page))
Then, go down til you see the box you wrote your siggy in, and click the blue writing under the box. on the new page, click the 'Browse' button and click on the file you saved the sig as. Then click 'Save Changes' or whatever it says. you will then have to copy the image code and paste it into your sig box