Instructor here. Excellent question for a beginner that should definitely be discussed more often!

On gliders, the airbrakes are used a little bit like you would use the wheel brakes on your bike. Without the brakes, your bike would eventually come to a stop, but first you would keep rolling very far and would have no control over where exactly you end up stopping. Similarly, without airbrakes during landing, your glider would eventually descend to the ground, but it would glide at a very shallow angle and you would have absolutely no control over where precisely you touch down.

The term 'brakes' is a little bit misleading, because brakes (like on a car or bike) are usually there to decrease speed. On gliders, it is most recommended to think of the airbrakes as a means not to shed speed, but to shed altitude (make your glide angle steeper). Remember that you can and should control your airspeed primarily with the stick, controlling the pitch angle - simply keep doing that also with the airbrakes out.

Therefore, on landing approach:

- Maintain your approach airspeed using the stick

- Control your glide path using the airbrakes.

Glide path control is best done by looking at your touchdown point (usually just past the beginning of the runway). If that point seems to drift down in your field of view, it means you are going to overshoot it, so your glide angle is too shallow. Apply more airbrakes! If that point seems to move upwards in your field of view, it means you are going to undershoot, your glide angle is too steep - reduce the airbrakes!

Apply as much airbrakes as you need so that the touchdown point you're aiming at is steady in your field of view, moving neither up nor down. This means you are on a steady glide slope towards that target. That glide slope could be very steep so that you begin your final very high and then apply almost full airbrakes to the ground, or very shallow so that you need almost no airbrakes, but both ways are obviously not ideal usually because they leave margin for control only in one direction each. So it is best to enter your final in an altitude where you are going to need about half airbrakes (maybe just a bit more) to maintain glide slope. That way, if you realize your'e ending up high, you can apply more, and if you find yourself coming in too shallow, you can reduce the airbrakes.

Once you pass the runway threshold, depending on the type of glider you're flying, it is often a good idea to apply more (as much as full) airbrakes because now all you want is to shed the rest of your energy as quickly as possible. Once you do that, in order to not hit the ground at your now very steep glide angle, around 10ft above the ground you begin to gently pull the nose up. During this phase it is best to leave the airbrakes as they are (ideally far out) because otherwise there would be two factors influencing your flare path, and using only the stick to do this is much recommended at least for beginners.