• the first one (~354KB) presents near the same example as the one below
• the second (~780KB) has been created by Stefan, an experienced user

In the directory you've chosen, select the files by clicking their checkboxes
(the "Sel" column contains the selection number following the order you've done it)

In the "Expression" combo, enter a renaming expression, validate it with the "Enter" key.
In this case, "%n" corresponds to the selection number and "%e" to the file name extension.
The "Future name" is immediately computed.

Click the button with the "Play" shape, the renaming is done.
The "Undo" button has become enabled.