I don't know if this idea will help anyone "learn" to retro hale, but as I think about it, If I concentrate on what I'm doing while retro haling, I find I'm doing something like this.
First I make sure I have my lungs about half full of room air before I start the retrohale.
Then I draw a mouthful of smoke from my pipe. At this point, my tongue is in a neutral position laying flat behind my lower teeth as much as possible.
With the smoke in my mouth, I start to slowly exhale through my nose, and at the same time slowly push my tongue back, towards the top of my throat, as if I was trying to touch my epiglottis (The dangling flap in the top posterior of your mouth, with the back of my tongue).
What I'm doing is, I am closing my mouth and reducing the area of the interior of my mouth, forcing the smoke into the back of my mouth and (hopefully), out through my nostrils, as I exhale.
I hope this description of how to do it isn't confusing, and I hope you will get the hang of it with a little experimentation and practice.
Good Luck!