What are the contents can be included in good LOR ?
Writing a good LOR depends on the programme you are applying for. As major such as MBA, PhD or MS require different style of LOR.
Here are few write-ups that I am sure will guide you through:
https://studyabroad.careers360.com/articles/lor-tips-for-ms-studies
https://studyabroad.careers360.com/articles/lor-tips-for-mba
https://studyabroad.careers360.com/articles/letter-of-recommendation-for-phd-students
While writing a LOR you can keep following points into consideration.
1) The language of the recommendation letter should be formal.
2) It is always better to write a letter of recommendation(LoR) on a letterhead.
3) It should give an overall picture of your personal characteristics, performance, experience, strength, capabilities and experience.
4) You must try to avoid vague statements