Here's a link to a "Furnaces for Dummies" type of file. It isn't specifically geared to your furnace, but will give you an overall understanding of how they work, what the components look like and where they're located. Hopefully it will give you a better understanding of what happens when the thermostat calls for heat and possibly, what's "not happening" in your furnace.
https://www.rvforum.net/miscfiles/Furnace_Trouble-2.pdf