Let the crank turn
In essence, they are "just" a wheelie performed backwards, except that because you ride backwards, the free-wheel engages with the crank, forcing the crank arms into reverse motion. Reaching the right point of balance for a regular manual is tricky enough to learn in one direction, but having to back-pedal into thin-air to follow the pace of the crank arms just creates more distraction and is rather unsettling. Only tons of practise will help you get that evasive trick, but here are a few tips.
Obviously, the fakie wheelie will only be possible with some backward momentum. This could be back-pedalling from a small endo, or back-hopping from a small step. But for the sake of learning, the least difficult start is to lean your front wheel against a wall and then energetically push yourself away from that wall.
Alternatively, on a flat, you could also start with a small endo to gain backward momentum, but the transition to fakie wheelie is harder to catch.
It does help if you launch this down a mild slope so that you continuously get some traction backwards.
Easier to start than to maintain
Never use the brakes!
In any case, never use the rear brake to control your speed backwards, a sudden stop of the rear wheel will throw you off balance on your back and even a progressive stop would deprive your senses from valuable pedal feedback.
Instead, control your speed by resisting a bit more on the driving pedal (whichever is positioned in front of the other). The hard bit is to maintain the same light pedal contact as you switch your front foot (from your good to your bad foot), without ever getting stuck on your bad foot.
So try to focus on following the pedals with your feet and you'll see some progress, from half a crank turn, to a full crank turn, to multiple crank turns.
Once you get used to backpedal in tune with the crank's motion, try to let your feet loose and focus more on your balance, for example, keeping your handlebars at a steady level.
This will be easier to visualize and focus on, and the whole move will look simpler to manage.
On a flat ground, without the natural traction of a slope, the fakie wheelie is a little bit harder to maintain. The second you stop back pedalling, you instantly lose your backward momentum, and too much pressure on the pedals will just shoot out the bike in front of you. Once you get used to a flat start, try your luck back-hopping from a small step as a way to build up backward momentum and land directly into a fakie wheelie.