At its most basic, you just replace the closing description tag, with
the new line and the same tag.
</description>
with
Created by Les</description>
But you might have to be careful, because if you have HTML in your
description, it probably has CDATA tags, so would be slightly
different, perhaps
]]></description>
with
<p>Created by Les</p>]]></description>
Alternatively if your placemarks have a common Style. Could just do it
with BalloonStyle...
https://developers.google.com/kml/documentation/kmlreference#balloonstyle