@skye precisely...

Also:

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Do not try to put things in their mouth whatsoever!

Communicate with them, even if they seem unconcious and/or not reacting to you. Update them on the situation by calmly talking to them.

Check their vitals. If unconcious but breathing, rotate them sideways. Make shure mouth and nose are not interrupted and that if they were to vomit (very unlikely!), it can flow out unhindered.

offer some still, non-carbonated water if they regain conciousness. Only offer something eat if they can sit upright and explicitly demand it themselves (a seizure burns a lot of calories, some may get hypoglykemia due to that so it's advised to communicate anything affecting blood sugar first!)

To deal with spectators, assign them tasks (i.e. on to call ambulance and/or direct EMS personnel to the site, stand around back towards the person having a seizure to deter spectrators and tell people wanting to film and gawk to fuck off.

Only restrict someone's movement if they'd hurt themselves otherwise (i.e. head banging against sth.) and only passively by cushioing them.

Brief EMS / Paramedics on the situation when they arrive so they can quickly take over. This should also be done so that the person in question - even unconcious - is potentially able to withness the situation and thus may be less frightened in the situation knowing they're being taken care of.

Yours faithfully,
a fmr. Firefighter