I am a 57 year old adult and I've just learned to ride a bike.
If that sounds like the beginning of an alchoholics anonymous meeting, there's good reason.

I just went for a short ride together with my daughter for the first time ever (she's sixteen now and I taught her to ride many years ago - ironic or what?).

Here's my experience, which I think gives a different perspective to those posting who learned at six and think it's all easy.

So this post is directed only at people like I was two weeks ago. Please don't bother to tell me my understanding of the gyroscopic effect is wrong.
It's still all new to me, I've only had about 6-7 one hour sessions so I am riding: but following a narrow path without falling in the bushes on either side and uphill starts are still beyond me.

If you are remotely like me you are not just learning to ride but also battling with fifty years of fear and shame.
We are older. Falling hurts and isn't just shrugged off with a cuddle (trust me the long chain mark going up my leg really hurts!).
So I would argue the best way to learn as an adult (and maybe for a child like I was fifty years ago) is by tiny, safe-feeling steps.
I've seen some programs that say "repeat ten times or do for five minutes".
IMHO this is rubbish: repeat until the fear reduces. You will then try a bit too hard and it'll go wrong and put you back a bit. Don't feel you can't go back a step or two. If like me you spend maybe an hour just sitting on the bike or scooting about a bit, that's all vital progress towards the goal.

I largely followed the ideas described in this forum and on other pages. I also found a uTube of an elderly lady learning that taught me nothing at all but gave me a kick towards starting. [BTW: I found internet pages on "how to learn to ride as an adult" - then took six months working up the courage to actually start]

Things that helped me:
The real eureka for me was being told that the handlebars aren't for steering but are used to balance.
The tip, turn away a little from the direction you want to turn is helpful, but only partially. [I still don't know how I turn! In the early days turning isn't in your repertoire anyway]
At the beginning I found it helpful to drop the saddle right down to have my feet flat on the floor, this boosts confidence immensely.
I removed the pedals, easy if you have the right spanner and know they undo in different directions. Find the right spanner for your saddle too.
I did my first four hours in the dark, not ideal but helps with the shame/embarrassment factor.
Like many posts, I started just by scooting about like this, this starts to give a feel of balance.
You next need a place to learn. I found "the perfect park": Wantage Park, UK in case you are near.
All these factors were helpful to me: wide spaces, flat parts and some relatively gentle slopes.
Few trees (although I did get very close a few times to the few there). But a few are good later when learning to steer round things.
The grass isn't too bumpy nor too long. (That said, some small bumps help learning as they stop you thinking about things too hard).
I found the grass helpful as I felt more in control on the slower surface than when on the the tarmac in the car park or paths (later).
A wide area is essential at the beginning, you are learning to balance, not steer.

I spent a bit of time on the flat and then freewheeling down the gentle hill, working my way further up the hill as I learned and gained confidence. This took me several sessions.
At this time you will need to learn how to stop: two tips, wait until stopped before putting feet down. Use both brakes gently, I found the front stronger and will start to send you over the front if you panic.
Look ahead not down.
Once freewheeling with a bit of confidence, practice lifting up your feet to where the pedals would be.
I found I could steer by looking where I wanted to go. Almost certainly anti-scientific but it may work for you too.

I expected the "putting the pedals back on" step to be hard but provided you still have the saddle low you will find scooting is still possible so isn't a huge jump (which we want to avoid).
The slope here was invaluable. Starting to pedal from a cold stop I still find hard, but starting to freewheel, and then lifting your feet up is easy if you have done the freewheeling practice.

If you're like me, this is the core to starting to ride. There's lots more (raising the saddle back up, going uphill, gears, going round stuff, playing with little slaloms) but I found this to be much more obvious than the first part above which is more about overcoming fear and "inner-riding" than anything else.

Hope this helps, Good Luck.
It's a hell of a kick when you move on a step!