Step 2: Select all the emails using the checkboxes, and press the delete key, or the trash icon to remove them. To remove more than the 50 shown on the first page, click on the option that says ...

Select All: The square with the square-ish arrangement of dots around it (far left icon), is your Select All button. Why on Earth that symbolizes selecting all is beyond me, but that’s what it is.

To mass-delete all your emails on Gmail, click the "Select" box, then "Select all conversations." Or you can choose specific emails to delete in bulk.