Plenty of sound advice, much of which follows what I was taught and used when I worked in a day centre and a special school. Fits were a daily occurance - all types.
Do the ABC check, but be careful when putting your fingers near the mouth. You are at risk of having your fingers bitten off. A jaw in spasm is incredibly strong. Also, the advice I was given was never to put anything in the mouth. They're unlikely to die from biting their tongue, but they are at risk of choking on a foreign object - even quite a large one. Tongue biting is actually quite rare.
If they stop breathing, then yes, attempt CPR if you know how to do it.
Otherwise, you should make them safe by moving away anything they could hit their head or limbs on.
Once they've passed through the spasm stage, they should be 'floppy' enough to put into recovery position. Obviously if they begin to choke before then, you're going to have turn them over and deal with it.
As Sara says, when they come round, they're likely to be embarrassed and probably quite frightened and confused. What they usually want then is to sleep. Stay with them until help arrives. If they're on their own when it happens, you'd be wise to get an ambulance whatever. After all, you have no idea how soon the next fit is likely to occur or how long ago the last one was. Fits that repeat in close succession can lead to a condition known as stasis (sp?) whereby the patient just goes into one huge, enfless fit and it can kill them.
If you can remember, try to note the time and approximate duration of the fit. It's useful to medical and ambulance staff when assessing the nature and severity of the fit.