There are 2 ways to add items to your Closet and Capsules.
1. In the Closet page, tap on the "+" in the bottom-right corner of each category. Choose the category of clothing, and use the Filter button in the top-right corner of the next screen to find what you're looking for.
If you are in an Active Capsule at the time, any new item you add to that Capsule will also be added to your Closet.
2. From the Discover page, tap on a suggested item, and tap "Add To Closet" at the bottom of the screen.