"The braking point refers to the moment the driver start applying the brakes for a corner. The braking maker is a visual reference chosen to help the driver achieve that goal accurately and consistently."
When searching for a braking marker you have to look around the track and see what visual clues could be used. On many tracks there are braking boards with symbols or distances to help you. In some cases the braking board is on the track and can be taken out by someone during a race and all of a sudden you do not know where to brake. That is why we look for other static visual clues that could be used instead. Examples of braking points could be the start of the kerb or fencing, a different in the texture of the track. Be very careful if you use shadows or skid marks as reference points, remember static cues is important.
Once you have found a braking marker you can now experiment with the quality of this braking cue. Let's say we choose a 100m board that is on the right as our braking point. We can now start by braking right before the 100m board and experiment with how late we can brake. If by braking before the board is too early we can move the braking point to when the car is aligned with the 100m board. If this is still too early, we can try and brake even later and choose to brake right after the 100m board and so on.
Let's say now that the 100m board was way too early. You can now try out the 50m board and see how it works. If you brake right before the 50m board and it is too late then you know that you need to brake somewhere in between these two markers. This continual fine tuning of where to brake is the fundamental skill required to set in motion how the corner will be driven.
Notice in the 'braking too early' image, the orange driver is braking a lot earlier than the reference lap. Easy to spot and the greater this distance the early they started braking. Using a different visual clue would be needed here as there is such a large gap between braking points. In the second image 'braking too late', the driver is braking just a little too late. This driver should experiment with fine tuning their visual cue and applying the brakes just a little bit earlier.