I guess it would depend on the situation.
Often times, the seizure is witnessed. Ask the bystander what happened. Sometimes the bystander will of assisted patient to ground, or bystander kind of "slumped" over, etc. I have also arrived on scene and been told by bystanders that the patient was aware a seizure was coming, and laid down prior to onset.
I myself would not put a c-collar on, because the risk of vomiting is to great and I would rather the patient not aspirate vomit. I also don't really see the purpose in trying to manually hold c-spine when someone is flailing around. It kind of defeats the purpose.
If it's an overly violent seizure, I would just fold up a blanket and put it under their head. But I haven't seen many seizures that are violent enough to the point that finding padding for their head was more important to me then trying to stop the seizure. I usually just move objects away from their head.
If it's an active seizure, follow your protocol (benzos) and do an assessment after. 90% of the time, the patient is postictal by the time I get there and I just roll them into recovery position in case they vomit until they start coming around.