Here is the procedure to clean junk files using CCleaner -
1. Once you start the app, tap on Analyse. It will start scanning your device for the junk files.
2. Once the analysis is complete, you will be presented with a list of what can be deleted. You may clean all by selecting the check box and tapping Clean, or you can check whether you want to delete all those files by tapping on Categories.
3. You can dig deeper in each category to mark the data that is safer to delete.
Make sure that you delete the empty folders which are created by apps and left as they are when you uninstall the app. Also check the downloads folder which may contain unneeded files.