Wear the frame and observe where the front part sits on your face. If the lenses are not aligned to your eyes and are too high up, you need to straighten the arms so that they slide towards the front of your ears. If the bridge of the glasses keeps sliding down your nose, you need to bend the arms so they hug the ears and prevent the frame from sliding. If the front part is not level, adjust one arm only.
For this step, you need to set the hairdryer to hot because you need hot air to soften the plastic frame. Point the hairdryer to the area where you want to make the adjustment. Keep it steady until the arm grows hot and becomes malleable.
If your glasses keep sliding down, bend the arms to keep the glasses up. If the glasses are sitting too high, push the bent parts of the arms upward to straighten them slightly to allow the frame to slide forward a bit and push the glasses down. Repeat this step until the desired positioning is achieved.