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Paradoxically, by using gravity.
From an aerodynamic standpoint a glider flies exactly the same way all heavier-than-air aircraft do. The wings of a glider turn (deflect) the air through which they're moving, producing the reactive force of lift which supports the glider. (Huh? It's not like that, you say? Actually it is…and many popular theories about lift are incorrect. But the devil is in the details, and exactly how turning occurs is a contentious subject even today.)
The paradox is that the glider needs to keep moving to produce lift, and this movement is provided by gravity. In effect the glider is continually "falling" forward; this falling creates airflow over the wings which produce lift, and the glider stays airborne.
It's an irony of soaring that the same force pulling the glider down is also responsible for keeping it aloft.