I had just purchased and printed my Eurostar train ticket (from St. Pancras to Gare du Nord) from their website. Since this is the first time we've used this system, do I just show the ticket agent my flimsy paper printout (with the bar code square thing), or do I have to get another physical ticket from a machine once I arrive at the station, just for good measure? Some advice would be appreciated, thanks

See the section of your printed ticket that states "HOLD THIS BAR CODE FIRMLY AGAINST THE READER ON THE TICKET GATE" near that squiggly thing? That's all you have to do. What you printed is your ticket, just like printing an airline boarding pass at home.

Don't worry. If the machine reader has any trouble reading it, there are usually attendants at the gate to help you.

Great, thanks a lot for the pointers

More advice for St-Pancras: There is a Marks and Sparks food boutique adjacent to the security check. Grab a sandwich and a bottle of something. Unlike airlines, you can carry it aboard for your (very) short journey.