There are basically two kinds of convulsion - the single convulsion which lasts for a minute or two and doesn't recur for at least 24 hours or repeated or continuous convulsions which are serious emergencies and need veterinary attention immediately.
In the latter situation, you should gently restrain the dog so it can't injure itself by placing a towel over it. A dog who is fitting should be left in a dark, quiet room. The owner should keep an eye on the dog without interfering in the actual fit as the more outside stimulus, the longer the fit can be.
Don't put your hand on the dog or in or near its mouth as you may get bitten and will require treatment yourself.Once you’ve restrained the dog, get to the vet immediately.
Single convulsions also require veterinary attention but are usually not so serious.
Again restrain the animal and get to the vet as soon as possible.