Polygon Crest is a tool developed by Soji for making addons for YSFlight. You can download it from Soji's website, ysflight.com.
There are some settings to setup prior to working with PolygonCrest.
This tutorial introduces you to PolygonCrest and shows you how to cycle through animations to test them prior to loading the DNM Model in-game.
Soji has kindly created some tutorials on his website about how to use PolygonCrest to model aircraft.