>>1626579There are mainly three possibilities:
-go to your local guides' office and ask if they offer courses/activities for beginners. Most guides' associations do offer such activities and also have the added bonus of providing/renting you the necessary safety gear or powder skis. Expensive, but this is the best option.
-do you know an expert backountry skier? Ask them if they can take you off piste for a couple of days and teach you the basics. I suggest you this route only if you know someone trustworthy.
-read books, watch backountry intro videos from guides, get the necessary safety gear and then start on your own with very easy off piste. Progress from there in a very conservative manner, preferably skiing with someone else. This is the least safe option for most people.