My dog had several fits and with her she started drooling, then just collapsed on the floor and started twitching all over. You can't do anything to stop it once they go into a fit but you should try to make sure they are in a place where they can't hurt themselves and when they come out of it try to keep them quiet and relaxed. They will lose control of their bladder and bowels so be prepared for a mess.
If your dog starts having fits you should get veterinary advice and do so immediately if it goes into a cycle of fits.