The University of New Hampshire offers great ways to help with selecting main ideas in class and while taking notes. Listed below are several tips, and the hyperlink to the University is attached.

 

SMI in class

1. Add headings to each section covered in class.
2. Pay attention to phrases that the professor repeats are notes as important information.
3. Ask other classmates what they thought was significant from the class lecture.

SMI while reading

1. Notice for the first sentence in each paragraph, it will usually tell you what the paragraph is going to be about.
2. Outline key terms and context, and write meanings of the terms.
3. Ask yourself what is the point after reading each section.

http://www.unh.edu/connect/selecting-main-ideas