First of all, you need to find on the left-side toolbar "Reports Builder" and click on it.

After that, you'll see the next window:

Here you can change the time frame of your report:

On the right, you can add various filters to your reports, such as advertiser (it's the same as App Object), campaign groups (folders), campaigns, geo, devices, inventory, and many other options. You can add one or more filters in your report.

In the same way, you can group your reports by different parameters like campaigns, inventory, device, time frames, geo, and so on.

You can click on "User Settings" and select the "General Metrics" option to display in your report such metrics as the number of impressions, clicks, CTR, CPC and reach. Or you can select "Video Metrics" to see video plays at 25%/50%/75%/100%. But the most important option is "App Metrics", where you can see the total number of installs, CPI, and IPM (installs per mille) of your campaigns.

But if you want to build your own report with the necessary metrics, you can click on "Customize" and add any metrics you want as on the screenshot below:

After you've chosen all necessary metrics, added filters, and selected group by option, click on "Create report" to build it.

If you changed some metrics/filters or group by option and want to refresh the report, just click here:

Once you decided, that report looks good, you can download it as a .xlsx file, clicking on this button:

Great job, your report is ready!