It's perfectly fine to let a girl walk by you and then turn around and catch up with her and open her. You just have to go direct. That's all. Obviously, you can't turn around and chase after a girl an then just ask for directions or something because that would seem disingenuous. So, if you're gonna go direct, just make sure you have a direct opener on the tip of your tongue ready for you, so you don't have to hesitate to think about what you should say. When I go direct during the day, I usually just say "Hey this is totally random, but I thought u were cute so I figured I'd come over and talk to you" OR "so I figured I'd come over here and embarrass myself my hitting on you innapropriately in public

"
So, don't worry if a girl walked by you. It's perfectly fine to turn around and chase her a little bit, as long as you go direct.
And if she already smiled at you, then you can open her with the classic "Hey, you can't look at me like that and not come over and talk to me."