To validate an interrail pass you simply need to enter the date of your first travel day on the pass. This effectively validates the pass and determines the end date (for 1 month passes).
You will need to validate and use your pass within 3 months of the date of purchase.
Eurail passes need to be stamped at the ticket office at your starting station (this can be done on the train by the conductor but they can charge a small fee for doing this).
Eurail passes are invalid if six months have passed since the issue date so be sure to time your purchase wisely!
Saver Pass - if you have a saver pass all of the people using the individual saver passes need to be present at the same time to validate them.
Select and Flexi Passes - as you have a selected number of travel days on these passes once they're validated you must also write in (in ink) the date on which you are travelling. Dates must be entered in the European date format, so 21st November would be 21-11. Make sure that you do this before boarding the train as the conductor may fine you assuming that you were trying to get a free ride!
The 7pm rule - If you're booking overnight trains make sure you know about The 7pm rule!
All passes please note: Make sure your pass is validated before your first journey as the conductor will assume that you were trying to travel for free and could charge you the full ticket price for your journey!