There is no right or wrong way. A summary of a book is highly personalto the reader. Just follow the 8 easy steps in the link below :)
When you have to write a summery of a book take the main points and flesh those out. Such as the climax, rising action, and falling action. those would be the best things to use when writing a summery.
I would suggest getting a pad and paper or open a word document and number the chapters down the left, leaving space for a few notes. On the first line next to the number limit yourself to identifying the main idea for that particular chapter and under that write/type two supporting details for this main idea. Keep this going for the duration of the text and when you are finished review your notes to get a shorter version of your text. From there decided the important details (major/minor details you mentioned) and work on making your summary to the point and well as encompassing the breadth of your text.