In the Season window, you will see the long tape of the magic Decoder. To read the diary, you need to complete the bar, and to fill it in you’ll need special tokens that you can find in the Manor. You can check the number tokens in the same window.
The progress bar will show the number of tokens you need to get each reward. An open book over the bar means that when you reach this mark, you’ll be able to decipher one or more pages of the diary. If you’ve reached a mark with an item, you’ll get it as a reward!
During the event, you’ll be able to read the pages of the diary at any time. To do this, just tap the "Read" button in the investigation window.
To get the reward you gained, just tap the “Claim” button. The rewards beyond the tape can be claimed only if you have a Detective Pass.