I left on a 3 day trip to paris on the 23rd April i left from bristol catching the train 2 paddington. changed onto the bakerloo line and got off at waterloo. that was easy and wasnt stressful at all. after you get off the tube you simply follow the dark blue signs towards waterloo international.
At waterloo international you colllect your tickets from either a touch screen computer or a ticket office (I chose the latter). After that you walk towards the check which you have to do at least 30 minutes before your departure. You put your ticket into the machine, then wait until it comes back out (about 2 seconds) collect it and walk through the automic barriers. Easy.
Then you walk through the automic doors and undergo a airport style security check, put all your belongings (bags, jackets, phones. etc.) into the machine, walk through the metal detector and pick up your belongings at the end of the machine.
After the security scan you already go through French Passport Control operated by the Police Nationale. (Outbound British Passport Control was abolished in 1994 for the EU and in 1998 in for all other countries). Then you look the departure times and go to your gate (platform) which is between 20 and 24.
you have to wait quite long time for the gate to open but all you do then is simply look at your coach number (1-18) and go up the escalator or steps to either the first 9 or the last 9 carriges.
You have to get on the exact carrige. Everyone is assigned a seating place and you have to sit in the one you have been assigned, which was a relief as the train was already full. After a few minutes the train leaves and after about 10 minutes of going at a relaxed pace through the south london suburbs it turns onto the CTRL (Channel Tunnel Rail Link) and whooshes through Kent. My train wasnt stopping at Ashford so it was straight to the Channel Tunnel I got a sandwhich before the tunnel from the buffet which is bit sparse. Instead they should have a proper dining car.
In the tunnel all the carriges are locked so you cant go to the buffet. The actual ride through the tunnel took about half an hour and France seemed a refreshing change from Britain when the train got out.
After that all you had to do was wait. After a while we stopped in Lille for a short time and a few people got off. Then the train proceeded to Paris, when you arrive there are no formalities whatsoever you just go.
On the return leg the signs at gare du nord towards the Check in are not very clear and it took about 5 mins to find. You already get the return tickets on the first leg. So unless you are a non EU or EFTA citizen like me you just go through check in exactly like at Waterloo though if you have non EU or EFTA citizenship like me you have to fill in a UK Landing Card. After Check-In you go through French Passport Control and then British Passport Control. After that you go through a French Customs Security Check. After that you wait for the Gate to open and then you get on the train like at Waterloo.
The experience on the train was exactly the same and once I arived at waterloo there were no formalities but we had to go through (literally) pasport control for travellers from Brussels.
There were no customs checks and I just left.
Overall: Amazing