Here's a basic framework with sample text. There's no need to adhere strictly to it; it should suit the situation at hand.
[Salutation. Always use a colon if writing to the US (commas are for friends and family). For the UK, use a comma or nothing at all.]
Dear Sir or Madam: (To Whom It May Concern: | Dear Admissions Office: | etc.)
[Intro paragraph. Basic introduction of the applicant.]
It gives me great pleasure to introduce to you a promising former employee of ours, Ms. Jane Smith, for admission to your graduate program in Finance. Ms. Smith worked under my supervision for two years as a telemarketer, and as such I am very familiar with her abilities and character.
[First body paragraph. More deeply introduce the applicant's performance.]
Ms. Smith started working with us in the summer of 2013. At first she was assigned to the position of junior telemarketer. But in six months, thanks to her exemplary performance, she was promoted to senior telemarketer. Such an achievement is rare among our telemarketing staff, and underscores her commitment to excellence, a trait that will serve her well in her proposed studies.
[Second body paragraph. Go into specifics about the applicant's job or classwork.]
I remember one time when Ms. Smith .....
[Third (and fourth) body paragraph. Add any additional pertinent information you think will help the applicant, but it must be within the context of your professional relationship. Nothing about home life, hobbies or other things a recommender should not be expected to know about. You can also mention a weakness, but don't dwell on it or make excuses for it; instead, present evidence on how the applicant has been overcoming or has the potential to overcome it.]
I would also like to mention the time Ms. Smith helped us organize our office Christmas party. ...
If Ms. Smith has any weaknesses of which to speak, it may be that....
[Conclusion]
Based on these facts, I am convinced that Ms. Smith has what it takes to thrive and excel amid the rigors of your demanding graduate curriculum. I am therefore proud to offer my heartiest recommendation in her support, and hope that you will grant her the honor of admission.
[Sign off]
Sincerely yours, (Warmest regards, | Respectfully yours | etc.)
John Jones
Telemarketing Manager
Acme Marketing Corporation