My advice based entirely on my own experience:
Choose an area with a guaranteed total absence of traffic if you possibly can. You’ve got enough to worry about without spooking yourself by thinking about the possibility of having to deal with a car.
Don’t count revolutions. Instead, focus on relaxing, feeling the balance, and enjoying the process. 20 revs with a relaxed attitude is better than 40 revs with a tense, gotta-beat-my-last-number attitude. Then again, maybe counting revs doesn’t make you tense up like it would me. Guess you have to be the judge of that.
Don’t look over your shoulder as you go. If you’ve chosen a suitable spot, you already know that it’s extremely unlikely that anything is gonna pop out and surprise you, so flying blind is no big deal. It’s in the very nature of backward riding that visibility is limited at best, so embrace that and learn to ride without looking. I think that frequently looking over your shoulder when you’re just learning sends a message to your brain that this is indeed a very dangerous thing which requires an abundance of caution, which is exactly what you don’t want. Too much caution = hesitation and UPDs. Also, riding backwards is very challenging and requires many hours of practice, and you don’t want to give yourself a crick in the neck.
You might learn faster on a smaller wheel, but if all you’re going to have for the foreseeable future is your 26", don’t let that stop you. I’ve never ridden anything but my 26" and am now almost as comfortable riding backward as forward, at least in a straight line. Just gotta keep at it.
It took me about three weeks to learn, but that was with relentless practice–three or more hours a day on most days.