There are few guidelines and points to be kept in mind while writing a letter of recommendation. These guidelines help the admissions committee to get a detailed insight on the applicant and the relevance of the recommendation. So when you ask anyone to write a recommendation for you, ensure it contains the following elements:

A brief explanation about their relationship with you. The letter should mention about how long have they known you and in what capacity have you interacted –advisor, mentor, faculty, teacher, supervisor or colleague. 

An emphasis on the observation being direct or indirect. It always has a positive impact if the observations were direct as that gives a clear understanding of the extent of interaction. For example, working directly is important and adds more value than being known through someone and having worked together remotely. Things like “I know her/him and observed them while working in a group headed by Mr. Y”. In this, the recommender only observed you working in another group led by someone else who is actually not writing the recommendation. 

LOR should carry a very accurate assessment of your eligibility and suitability for the course/program/study. Emphasis should be more on suitability rather than the recommender sounding more of like an advocate. For example, the recommender should mention how you fared in class about a relative course of interest/ rank position like top 3 or top 1% of the class/university. 

A recommendation letter should never mention any grades or scores of competitive tests unless required to interpret some academic details. These are already mentioned in your application and repeating them in an LOR is a big No.