Presposition "of"

Hi,
Sometimes i have hard time describe things and also see people do that in school. I wonder if it is acceptable and grammartically correct; for example, there is a description about statistics from a text, and I am not if it is easy to read, or can be revized," The value of a variable for one particular element from the sample or population is called an observation. A data set consists of the observations of a variable for the elements of a sample."

Also, sometimes when I describe procedures on repairing things, like "turn on the button on the right corner of the back of microscope body." Are they correctly expressed?

Thanks,

Ryan