Short Answer 
To write an effective reference, first define your role related to the applicant and how it adds credibility. Then, describe the nature of your working relationship and the projects you collaborated on, followed by specifying the duration of your working relationship to provide a context for your evaluation.
Step 1: Define Your Role
Begin your response by clearly stating your job title and position in relation to the applicant. This could be as their direct supervisor, a colleague, or even a client. Providing your title adds credibility to your reference and helps to frame how you viewed the applicant’s work and skills.
Step 2: Describe Your Working Relationship
Next, elaborate on the nature of your working relationship. Discuss the specifics of how you interacted with the applicant, outlining the tasks and projects you collaborated on. Highlighting these details allows the person requesting the reference to understand the context of your evaluation and the capacity in which you observed their performance.
Step 3: Specify the Time Period
Finally, mention the duration of your working relationship with the applicant. Stating the time frame during which you worked together will give the reference requester a sense of how well you know the applicant’s abilities over time. It helps to note any significant achievements or improvements witnessed during that period, which reinforces your assessment.