First of all, we have to define a place for where the bubble can be placed. Several techniques can be used; I find that a measuring tape works just fine.

 

Secure it from both ends so it hovers right above the water. Placing here is also a factor, you should put it one third of the distance from you to the end of the tray. This allows you to get more breathing space in the top of the bubble, where it really adds to the composition of the photo.

 

These markers will also be used for you to guide the bubble when taking the shot. Even your breathing causes the bubble to shift position ever so slightly, and you'll know that between the markers the area is in focus.