First thing, set the erasing method. There are several ways to securely overwrite files. Some methods are better or faster than others.
Click Edit>>Preferences>>Erasing. Select the Files tab. For erasing files, choose Gutmann.

Each overwrite of a file is called a pass. The more passes, the more thorough the deletion. The Gutmann method makes 35 passes. That exceeds military standards. You can be sure your files are securely erased.

You can also use Eraser to overwrite unused space on your hard drive. This setting does not affect the files you want to keep. So, only the files you want are accessible. That unused space could be dozens of gigabytes. Using the Gutmann method would take a really long time. Other methods do a decent job. And they won't waste a lot of time and system resources.

Click on the Unused Disk Space tab. Again, you'll see the list of erasing methods. Select either Pseudorandom Data or one of the US DoD options. These methods make far fewer passes than Gutmann. Click OK to confirm the settings.

OK, now we can start erasing files. Eraser has two modes, On-Demand and Scheduler. Scheduler lets you set recurring deletions. We'll get to that in a second. First, let's look at On-Demand. This is used for one-time deletions.