If you have uploaded or imported all of your content into the "My Content" area of the dashboard, you now will want to fill your app with all your desired content.
You should have already organized the framework of your app, including adding all potential Sections and Subsections. If not, take a look at these articles first:
Navigate to the Section where you want to add content and you should see something similar to this in the dashboard:
Press the +Content button, and you will be prompted to browse through all the existing content you have added into the dashboard. You can add as much content as you want into your Section by pressing the Select button next to your chosen content.
Complete all the required details (you can change them later) as shown below (screenshot shows the settings for an RSS feed). You can choose whether this is free content, paid, requires a registration wall, etc. and make sure "Active" is selected if you want it to show up in your app.
Repeat these steps until you have all the content loaded into your app Sections.
For more details on how to add content, check out these articles: How to Add Content