Opinions about what works varies a lot. In the end what matters is finding what works for you. Persistence is the most important skill. Keep trying different things. You brain will work it out eventually. Just don’t give up and you will get there.
Start by sitting on the uni holding a support for a few minutes to get a feel for pedaling and learn how to steer the wheel by twisting the uni between your thighs.
Don’t spend long on the fence and start trying to ride into the open from a support or backstop. The fundamental skill is to steer the wheel under your fall. A fence or wall gets in the way.
Try thinking “put the wheel where you would put your leading foot if walking”. This can employ some of your established neural pathways. Of course riding is far more complex but it may provide some foundation to build on.
Have the seat as high as you feel confident that you can catch yourself in a sideways fall. This will be below the optimum for riding but it is better than being terrified. Fear is a great inhibitor to learning. The best height for a beginner is a compromise and depends on confidence.
Aim to get your weight on the saddle but start with most of your weight on the pedals. The uni will pop out from under you if you try to sit before you learn to keep the uni under you.
Try gripping the saddle between your thighs to stop it slipping out. If you have the saddle too low your thighs will move around too much to grip it. You will just need to be braver and raise the seat if too low is the problem. It is very difficult for a beginner to ride with a low seat.
Most novices start by sitting balanced on the uni and start pedaling. The uni moves forward and the rider stays still. Only one outcome from that and for most it is their last attempt. Falling backwards is very unnerving. Much better to err in the direction of leaning too far forward.
To ride away, lean both you and the uni forward until you are nearly falling then start pedaling rapidly to bring the wheel under you. This gets you quickly up to speed. Like on a bike, riding slowly is not a beginner’s skill.
The first thing to learn is the emergency dismount, running out of a fall. It is all you will do for very many attempts and is important practice.
Falling is a constant companion while learning. Make friends with it from the beginning. I recommend starting on smooth thick grass with a slight downhill slope. You won’t be able to ride straight away anyway so best optimise for falling.
Once you can manage a couple of turns move onto a smooth flat surface.
When you actually manage to get moving, don’t sit up too straight. Lean your body slightly forwards at the hips but don’t hunch. When you are in the right position, the uni will be leaning slightly back and your centre of mass will be above the contact point of the tyre on the ground. The uni is far more forgiving of imprecise positioning than when upright.
Try to keep you leg movements in line. Thighs are heavy and flailing them about doesn’t usually help your balance.
Wear protection, especially on the wrists. A cheap set of skateboarder knee, elbow and wrist guards is a good investment. Wear sturdy boots that cover the ankles because they can get whacked. This reduces the movement in your ankles too, removing one of the degrees of freedom in your movements, making it simpler for your brain to grasp.
The technique required by a learner is radically different from that of an accomplished rider. If you have access to an experienced rider, get them to show you how they ride off road and try to copy that on a smooth surface. They will use a style that affords increased stability and this is what you need to emulate as a beginner.
As you get better you will learn to sit on the seat with the uni in the much more responsive more upright geometry. But first you need to learn how to ride badly.
If it gets frustrating (it probably will), take a day off practice. Many riders report surprising improvement after a break. It give the brain a chance to consolidate what it has learnt without more overwhelming stimulus.