France Rail Pass Validation

You can buy an activate it online or at the train station ticket window

It is normally done by staff at an SNCF ticket office. Don't write on it yourself before that happens. https://www.ricksteves.com/travel-tips/transportation/trains/using-your-rail-pass In addition to train station locations, SNCF boutiques in larger cities may be a handier stop.

Think twice about buying these passes, they are almost never a good deal these days given the mandatory long distance reservation costs and the limited number of 'pass available' seats on TGV and other long distance trains.

As I've mentioned before, there's no longer supposed to be an artificial limit on passholder seat reservations within France - only on international routes.

You can have that rail pass activated at the ticket counter in a train station. The staff person at the counter will stamp it showing its date and the words, SNCF. After that you're in business Yes, get that done prior to boarding the train.

This can also be done in the ticket office, which is what I do in Germany by going to the Reisezentrum in a train station.

I have found no French itinerary making pass purchase a worthwhile option. The TGV reservation fee can cost about what advance purchase tickets cost and even if there is no artificial limit on passholder seat reservations, there are trains which sell out, particularly during holiday periods.