Book train reservations before arrival

Recommendation says allowed to book seat reservations one-month before Jr Pass use. How can I do that if we have not exchanged passes in Japan

Travel arrival - my family will arrive at HANEDA airport on February 29 @ 7:45pm.

3/1 - Shinagawa to Kyoto/Nara
3/2 - Shinagawa to Kobe or Hiroshima
3/3 - Shinagawa to Osaka
3/4 - Osaka to Tokyo
3/5 - Osaka to Hakone
3/8 - Osaka to Sapporo
3/11- Sapporo to Shinagawa*

We would like to reserve train seats for party of 10.

Please advise

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