When you first visit the Custom Clean function of CCleaner, you will be presented with the following screen:
From here, you can customize your cleaning options by selecting or clearing the check boxes, or you can stick with the default settings. The options that look like they are greyed out can be selected, but are not done so by default.
Please note that you can check in both the Windows (selected by default) and Applications tabs at the top of this list to switch between the two lists of cleaning options.
When you have decided what you want to clean, just click Analyze. This will allow you to see how many junk files are currently taking up unnecessary space on your computer. The analysis should only take a few seconds, and will display a list of applications that are taking up space with details of the files responsible.
If you want CCleaner to remove these files and clean the specified areas, click Run Cleaner. Afterwards, you will see a summary of the applications cleaned. Double-click a specific application's name in the Cleaning Complete list to see exactly which files were removed.
Please note that it’s entirely normal for there to be a small degree of variation between these two sets of results, due to files being created or deleted automatically in between the two steps. The Cleaning process runs its own analysis to determine what files should be cleaned at that moment.
In fact, once you are confident that the cleaning options are set exactly the way you want for day-to-day use, you may skip straight to the Run Cleaner button if desired - it never hurts to analyze first, though.