Angle in from the side. Use the "side hug" approach, especially in the beginning. It's a safe bet when making moves on a tall girl. Try not come across as a creepy.
Avoid burying your face in her bosom. Turn your head away from her. Gently squeeze her.
Hug her from behind. Walk up behind the tall girl and wrap your arms around her. Place your arms securely beneath her chest to avoid unintentional groping.
Level the playing field. Stand on an elevated surface. Give yourself some height with the help of a staircase and let the hug happen naturally. Take advantage when the opportunity presents itself, like while you're walking down the stairs together. Step on the first stair while she is still beneath you. Turn around and hug the tall girl when you are standing eye-to-eye. Be subtle and spontaneous.
Ask permission. Request politely for the tall girl to bend down. Feel comfortable in your own skin and acknowledge the obvious height difference. Reach up and wrap your arms around her neck. Be sure that your head is safely elevated above her breasts before you hug.
Relax and be comfortable. Stop worrying about it. Uneasiness makes you both feel awkward. If the time is right, you'll know it. Hug the tall girl when the right moment arrives.