Please don't worry.
The shuttle itself is very straightforward. You will get your ticket hanger showing your time of boarding when you arrive and check in. You then park up at the terminal waiting for your boarding time.
Keep a close eye on the departure board because things move fast - don't wait for the spoken announcement.
When your train is called, get back in your car and follow the well-indicated route to passport control. There can be quite some delays here so as soon as your train is called to board, go immediately. There are two passport controls to clear - UK and France.
You are then directed to the boarding lines. Follow the line that says >1.85m if that is your case.
There will be a person directing you to the right boarding line - you can ask any question if you are confused.
Then wait and board.
It is easy to drive on - just follow the queue.
There are toilets on the train but it is better to go at the terminal - sometimes the ones on the train are out of order. You can get out of your car but your car is the only place you can sit down. The crossing takes 35 minutes - it will go in a flash.
Driving on the French side is calmer and the motorways are excellent.
Just tell your passengers to be quiet at any point where you need to concentrate. Alternatively give them tasks or things to look out for, e.g. watch out for your letter so you can board / tell you when the light goes green for you / look for the white horse carved on the hill - a lovely landmark to watch out for at the tunnel.