Input the page number of the first page AFTER the page where melody wrote her first message. She wrote her first message on the on Wednesday, January 6th 2021 and the message text was "They are watching me. Always have and always will be. I have become so paranoid. -love melody" so you will have to search for that.
IMPORTANT: page numbers are displayed in 15-steps, you can see the current page number in the current URL and it looks something like "start=810" or something, 810 being the page number of melodies earliest message at the time of writing this. So you would have to input 810 down below and then press the button.
After you have clicked the start button a dialog will prompt you to choose the path where the archive will be saved. Choose a path and then it will start. You can see the progress on the progress bar that will appear after the application has started.