Polly will only ever go into a seizure while she is asleep, so there are no warning signs and typically they will happen in the middle of the night. As she makes quite a distinctive choking/coughing sound I always wake up when she has one.
Here is my advice of how to manage a seizure:
1) Recognise and remain calm
It is very distressing when you see your cat having a fit but you have to remember that they are unconscious during a seizure, so they are not in pain or aware of what is happening. Try not to interfere too much during the fit. I have had occasions when her claw has been caught on something and I've unhooked it to prevent her getting hurt. Don't try to hold them down, just let the fit ride out.
2) Establish the end of the seizure
Learning when the fit has ended is very important as this is the point you can now pick up your cat and begin comforting them. The convulsions will slow down and become body twitches and they will be staring with their pupils dilated. At this point after a fit I pick Polly up and cradle her like a baby. Cats are usually temporarily blinded after a fit and this is why their pupils are dilated and they just stare off into space.
I have found that Polly recovers quicker after a fit if she is cradled like a baby in a dark room and spoken to in a soft calm voice. Hearing my voice is very reassuring while she is temporarily blinded and being in my arms makes her feel safe. She is often still having body twitches at this point and this stage can last for 1 - 5 minutes. As soon as she is ready to be put on the ground she wriggles. Once she is put on the ground she will pace the house for a few minutes. I usually continue talking to her during this stage.
4) Leave them to it
After the pacing stage Polly will always go and eat her biscuits. This is the point that I then leave her to it and try and recover myself from what is very stressful for me as well as her. You may be worried but its very unlikely your cat will have another fit so soon after, so try and relax and distract yourself.