On which part of the descent does a pilot begin to flare?
This answer is a bit dated since it applies to 747-100 and -200 aircraft in the 1990s. However, the same principles still apply, and I'm sure the same technique would work now on current 747s. Whether or not anybody uses this protocol now I do not know.

We flared the aircraft by the numbers without reference to peripheral vision. The use of outside vision was, of course, critical for runway alignment and any necessary crosswind corrections.

We flared with reference to the radar altimeter. If it was a 'talking' altimeter, it would announce the altitude of the main landing gear above the ground in the following increments: 100, 50, 40, 30, 20, 10, with the increments being in feet. If the the radar altimeter was an old non-talker, the flight engineer would read those increments.

Coming down the typical 3 degree glideslope on a stabilized approach, the nose would be 5 degrees up, and the fuel flow would be about 5,000 lbs per engine. When you heard the 100 foot call, that served simply as an alert. When you heard the 50 foot, you pulled most of the power and raised the nose 2 more degrees. When you heard the 10 foot call, you pulled the rest of the power while raising the nose an additional one degree, maybe a little more if you had the runway length to try for a real greaser at the risk of floating. Then you just waited. Touch down of the main gear would occur within 1 to 3 seconds. If it didn't occur within that time frame, you were floating and would want to reduce your pitch a degree or so.

These 'numbers' would be adjusted if you were making a non-stabilized approach, which we often did back then when conditions permitted because stabilized approaches used more time and fuel. Plus, a non-stabilized approach is more challenging and thus much more fun. As I understand it, they're frowned upon these days even to the point of ops specs requiring a stabilized approach.