How do I activate my Japan Rail Pass once in Japan?

In order to activate your Japan Rail Pass you will have to exchange your voucher at a JR office. These offices can be found at the airport or in the main train stations. To check the list of the JR Exchange Offices and their opening hours, please consult the following page: https://japanrailpass.net/en/exchange.html

At the office, fill out the form given to you and show the "temporary visitor" entry stamp that's in your passport. When exchanging your voucher you will be asked to decide on the date you want to start using your pass.

The exchange date does not have to be the pass start date. You can choose your start date when you exchange your voucher, but the date cannot be more than 30 days after the exchange date. 

Once it's validated, you will be able to use your JR Pass starting on the date you've chosen for the consecutive number of days for which your pass is valid.