There will be two places to do it from.
The 1st is from the main reservation screen. If you swipe up, you will see 5 tiles (Port Adventures, Spa & Fitness, Adult Dining, Onboard Fun and Nursery). If you touch one of those, you will then select which day you want to book for.
The 2nd place is to touch "My Plans" at the bottom of the screen. In this method, you will touch "Add Activities" for one of the days and then select the type "Spa, Dining, etc.).
The app has been a bit flaky lately, so you might need to switch over to the
DCL website.