Step One:  Approach Ball Girl and initiate conversation.  

Start by watching her from afar.  Does she have low enough standards to consider you? Probably not, but it may still be worth a shot.  Slowly approach her from behind while she's on the job.  If she's working she can't be rude to you or run away.

Use your body language to signal that you are comfortable and confident.   It is integral to get down to her level before you get down.  Bend over and lean on the wall to show how just comfortable you are.  Eye contact is critical.  Be sure to spread feet in wide stance to get maximum balance.   Don't forget to relax and smile.  There's nothing to be nervous about. Just raise your rump high into the air like a baboon. Say something clever and non-threatening like "You must really like to touch balls!" Then let out a huge belly laugh, that will surely set her at ease.

Step Two: Prepare to fight off your buddy who is trying to c*ck block you.

When she stands, follow her lead, never breaking eye contact.  Take an open stance so you can fight off other interested males while still giving her the appropriate attention.  Once the other males know that they cannot sneak up on you they'll likely surrender the prey. Never take your eyes off of her, staring is an important part in the mating ritual.

Step Three:  Hint at your romantic intentions.

Once all competitors have been scared off, step directly in front of her line of sight and flex your pectoral muscles.  Alternate between left and right if possible.  Sway hips forward and backward ever so slowly. Make sure that the fly on your pleated slacks is open to give her a peek of your goods.

Step Four:  Close the deal.

Ask her if she'd like to sit on a comfortable padded wall.  When she has exposed her back to you she has submitted to your advances. You may now put your arm around her or feel her up.