Once you filled out the report and click Continue, you can add any files that may help the developer, like screenshots. Feedback Assistant will attach a system profile and a compressed folder of diagnostic files.
In the final screen, you can review your report. Click Submit when you're done. Congratulations! You did your duty toward building a better OS. Keep up the good work!
Using the Feedback app in iOS 9
Before reporting a bug in iOS 9, take a few steps to see if the problem is reproducible. Say you're having trouble tapping the tabs in the Twitter app. Try force-quitting the app (double-tap the Home button to get to the app switcher, and then swipe up on the Twitter preview to quit the app) and relaunching it to see if that helps. Ditto with restarting your phone. It might not help, but at least you'll be able to say in your report what fixes you tried.
If the problem is with a third-party app, check the App Store's Updates tab to see if there's an update for the app in question--no point submitting feedback for a bug that's already been fixed. You can submit a screenshot with your report, so if the bug is visual, go ahead and snap some proof! This won't make sense for every bug, of course--it's pretty impossible to get a screenshot of an app crashing.
The Feedback app is pretty bare-bones, opening to a Feedback Assistant screen with the same kinds of folders you'd see in an email app: Inbox, Drafts, and Submitted. (Inbox is where Apple can leave notes and announcements for its beta testers, i.e. you. Right now all that's in there is a welcome note.) To start a new report, click New Feedback, or the compose button in the top-right.
Then you're basically just filling out a form. Be descriptive and thorough as you answer each question. In the Basic Information section, for example, you'll be asked for a descriptive title. Be specific! Don't just call it "Bug" or "Crash." Instead, flesh it out a little to something like, "Apple Store app crashes on launch."
At the end of the form, you'll get a chance to add a screenshot or video, and if you'd like you can click the crash log attachments that are added automatically to give Apple's engineers a more complete picture of what happened. After a reminder that the logs are being attached, you'll tap Submit in the upper-right. Then pat yourself on the back for doing your duty as a beta tester!
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