In reality, there isn't a definitive way to convince a girl to let you touch her boobs, but there are some ways to make it more likely that she'd want you to.
- Be yourself. If there's one thing women don't like, it's men that are pretending to be something or someone they're not. That's a pretty likely way to halt any progress in the boob touching department.
- Pay her complements. Don't just tell her she's pretty over and over again (although that's good). Be creative, and tell her honestly if you think her eyes look nice, or she looks great in her outfit.
- Take your time. Don't go in with the intention of physical encounters and nothing else. Not many women will be up for this, and if they are, you'll probably already know it. Relax, and enjoy each other's company.
Overall, don't be too pushy, and take your time with things. If it's meant to happen, it'll happen. That being said, a few complements here and there and a genuine man in front of her probably won't slow things down either.
If you want a bit of further advice, you'd probably better get a pen ready, and watch this video!