To better understand what's happening down at the tire's contact patch when you combine braking and cornering, we can look at the actual forces involved. On a street bike with treaded tires, it's common to see 1g of maximum cornering force and 1g of maximum braking force. It's no coincidence that these numbers are close to, if not exactly, identical: For the most part, the tires' available traction is what sets these absolute limits. If the maximum force the front tire can withstand is 1g, it is capable of that level in cornering or braking - but not both together.
When cornering and braking forces are combined, they sum using vector addition and that total force must remain below that same maximum - 1g in this case. If you remember high school math, vectors add using the Pythagorean Theorem: When you walk three blocks east and four blocks north, the total is five blocks as the crow flies. For a trail braking example, if you are at .6g of cornering force (or about 30 degrees of lean angle) and .8g of braking force, the vector sum is approximately 1g. At any stage of the corner entry, from initial turn-in to maximum lean, the vector sum of cornering and braking forces must be below this maximum; as the bike is leaned more into the corner, the brakes must progressively be released.
What we are looking for when trail braking is that smooth release of the brake as the bike is leaned over more, so that the vector sum of the cornering and braking forces on the front tire is constant and close to the maximum capability of the tire - in our example, the total combined force would ideally be 1g from the instant the rider gets on the brake and right through the corner to the exit. This means that as soon as you lean into the corner, you must begin to release the brake, and that the brake must be fully released just as you reach maximum lean.
In my experience, most riders don't work on trail braking specifically. Rather, they work on braking later, which in turn forces them to hold the brakes on while they lean into the apex if they want to make the corner. It's a roundabout way to learn, and brings panic and the risk of a crash into the equation. A better option is to brake and turn in at your usual markers, but gradually add in the trail braking by holding the brake on as you begin to turn into the corner, progressively more each lap. The resulting corner speed will definitely be slower than your usual, but there will be less panic and less chance of a crash. Work up to combinations of more brake and lean angle, moving your braking marker appropriately as you get comfortable with braking deeper into the corner.
There are a number of ways to monitor trail braking using data acquisition and track progress. One is to look at brake pressure and lean angle data and how they are combined during corner entry. Another is to look at GPS data and the actual braking and cornering forces involved, and their vector sum. Both methods are useful, but show quite different detail: The former shows the rider's inputs and what is happening at the controls, while the latter gives a picture of what's going on at the tire - the motorcycle's output.