As Aviatrix said, your railpass is your ticket. With your flexi pass, once you arrive in Europe, go to a train station and "validate" it so that it is basically activated. On the day you want to travel by train, just mark in the day and month in the appropriate slots on the flexi-pass and you can hop on and off as many trains (not local metro's though) as you like for most part. You just wander around and find a seat. If you can't find one, you just have to hang out by doors or somewhere out of the way.
The "catch" is some "specialty" trains (like the high speed trains, some overnight ones, etc) require you to make/buy a reservation ahead of time and pay a supplemental charge. Case in point, I think you mentioned that you are going from Amsterdam to Paris so I'm assuming you'll need to take the Thalys. If you bought a reservation on the Thalys without your Eurail pass, it would cost you about ~$150. If you buy a reservation with your Eurail pass, it would only cost you about ~$20 (very rough).
If you have to make a reservation, you don't have to buy it through that website. You can just go to the local train station and buy it day of; the risk of buying day of, of course, is that they run of of seats/reservations. It's a PITA to change reservations and I don't think you can a full refund back.
There are typically multiple journeys to the same destination so if you can't get on the one you want, you'll likely be able to get on a later one.
Websites we've used info on train travel are:
Rick Steves (
http://www.ricksteves.com/rail/rail_menu.htm)
The Man in Seat Sixty-One (
http://www.seat61.com/index.html)
For schedule info:
http://www.bahn.de
But for more up to date info, go to website of the national train lines which you identify off The Man in Seat Sixty-One web sites.