Reserving seats using the JR Pass

Hello,

We will be travelling around Japan between 23 December 2013 - 4th Jan 2014. We will buy the 7 day JR pass for travel between 27th December 2013 - 2nd Jan 2014.

Since we are out of the country (landing into Japan only on 23rd December 2013), we just wanted to check as to how we could reserve seats on the our trains esp the bullet trains (shinshaken's). We understand that the last week of December is peak travel season in Japan and we would really like to book seats for our Nikko-Tokyo-Kyoto-Osaka legs.

Any help will be appreciated.