If you are in position where the camera angle is directed up 2B's dress, 2B will turn around suddenly and the camera angle will change. If this occurs ten times, the achievement will unlock. There are times when this might happen inadvertently especially when 2B is ascending elevated terrain. It can be done deliberately on level ground.
The earliest point where it can be unlocked is in the the prologue right after you get control of 2B. So, for Bean Divers, this is the achievement that can be unlocked earliest in the game
although you can wait till much later to unlock it. You can do it immediately upon gaining control of 2B but it's particularly easy after you progress a few steps across a walkway that 9S lowers. There are some containers. While 2B is standing on the edge of a container, position the camera angle to look upwards under her dress. It takes a couple of seconds
. When she abruptly turns, you know that it has worked. Do this ten times to unlock the achievement.Edited to add 30 July 2018
. As the discussion with @Karachi King
in the comments below indicates, it is important to hold the camera steady. This is probably more important than looking up 2B's skirt. The important thing, assuming that you have a low camera angle, is to hold the camera steady. As stated in the above paragraph, it takes a couple of seconds. Do not continue to search around for the best view. Camera movement prevents 2B's reaction. Instead, get a satisfactory angle and hold the camera steady. Only if 2B ignores you for a couple of seconds should you try to change the camera angle and try a different view.Note
: If the achievement is too sophomoric for your tastes, it can be purchased for 50,000 gold in the secret achievement shop. This special shop becomes available after three playthroughs. By that time, though, it is quite likely to have popped accidentally even if you have not been deliberately working towards unlocking it.