Seat reservations online

Hi.
We have bought and paid for online a pair of 7 day JR-West All Area passes, and have received a reservation number and he know how to pick up the passes (as we come through Shin-Osaka).

However, my wife and I are confused about how we are able to pre-book online the up-to 6 seat reservations that apparently we are allowed. I know how to do this at a ticket office, but we'd like to do this in advance.

Any guidance would be much appreciated.