Install QGIS from QGIS.org.

Load QGIS and go to Plugins -> Manage and Install Plugins.

Under the All tab, search for "encoded polyline" and click Encoded Polyline Exporter (your interface may differ slightly).

Click the Install plugin button and wait for installation to complete (an icon will appear on your top toolbar and a new Encoded Polyline entry under the Vector menu will be created).

For information on using the Encoded Polyline Exporter plugin, read the How do I use the Encoded Polyline Exporter plugin for QGIS? page.