The whole "practice makes perfect" quote is somewhat true, however, ineffective practice is and will always be ineffective. Here's some pointers:
1. Start in a slow, easy to control car and build yourself up over time.
The reason for this is simple. When turning off the braking line you need to use reference points on the track to spot your braking zones. Things like brake board markers, trees, shadows, environmental objects and so on. In a slow car, it gives you more time to initially look around and pick out these reference points rather than a fast car where it's just harder to react.
2. Pick a track with that you are comfortable and know well.
Self explanatory. If you pick a track that you don't like or know well, you'll be more inclined to give up simply because not only are you learning to braking points but you're also learning the circuit itself. It'll become frustrating if you make this mistake.
3. Try to pick a circuit with a mixture of all types of braking zones.
This is once again very easy to understand. I personally think Yas Marina and Road America are fantastic tracks because they have corners that require harsh braking zones, trail braking, lifting off the throttle, sharp corners and long flowing bends as well.
4. Watch top leaderboard times and tutorials online.
You'll learn a lot from the very best from this game. Not only will you learn braking points but you'll learn techniques and a whole host of other things as well.
5. Just keep practicing and when you feel comfortable, switch tacks and cars.
Once you've mastered one track in one car, try a faster car or switch tracks all together. I would however always stick to one car that you really like for learning all tracks initially. Pick a car that has really good braking and handling. Something light and grippy like an MX5 or an Elise is perfect.
Here's a video of me at the Nurburgring in a McLaren P1. The footage is live, ignore the terrible FPS, it was before I got a capture card. I run with no HUD, no assists (apart from switching between manual and manual with clutch depending on the car) and I'm on a wheel. It may look really hard at first but trust me, once you get used to it you'll never go back and not only is it very rewarding, but it's actually surprisingly easy once you've just put in the time and effort.
https://youtu.be/XMD8MTLG6fEHope this helps!