Purchasing a jp rail but booking for the busy period - how to book in advance

Good afternoon,
I am looking at buying a 21 day JR Pass for when I arrive in Japan December 30 and will be there for a few weeks with much of my travel early in my trip by train.. I have read that it is a busy period and you need to book the bullet train in advance. But also that I cannot activate the JP rail pass until my arrival. My question is how do I book multiple trips in advance on the bullet train to ensure I get a seat during this busy period prior to my arrival in Japan?
Kind Regards,
Kai from Australia
Apologies if the below translate is incorrect*
こんにちは、 12月30日に日本に到着するため、21日間のJRパスを購入しようと考えています。数週間滞在し、旅の大半は電車での旅行の早い段階で行う予定です。今は繁忙期だと読みましたが、あなたは新幹線は事前に予約する必要があります。ただし、到着するまでJPレールパスを有効にすることはできません。私の質問は、日本到着前のこの混雑した時期に確実に座席を確保するために、新幹線の複数の旅行を事前に予約するにはどうすればよいですか?
敬具、 Kon'nichiwa, 12 tsuki 30-nichi ni Nihon ni tōchaku suru tame, 21-kakan no JR pasu o kōnyū shiyou to kangaete imasu. Sū-shūkan taizai shi, tabi no taihan wa densha de no ryokō no hayai dankai de okonau yoteidesu. Ima wa hanbō-kida to yomimashitaga, anata wa shinkansen wa jizen ni yoyaku suru hitsuyō ga arimasu. Tadashi, tōchaku suru made JP rērupasu o yūkō ni suru koto wa dekimasen. Watashi no shitsumon wa, Nihon tōchaku mae no kono konzatsu shita jiki ni kakujitsu ni zaseki o kakuho suru tame ni, shinkansen no fukusū no ryokō o jizen ni yoyaku suru ni wa dōsureba yoidesu ka? Keigu,