Simply replace the unfilled areas such as "" and "[date]" with your information.
As you know, I will be applying to the master's program at [School] of [Field] at [University]. I plan to pursue a degree in [field] with a long-term goal of [long term objective]. It is a field I first became interested in when [very brief explanation of goal background].
As you have been a [role] for me, I would like to humbly ask for your letter of recommendation. If you feel that you would be able to write a supportive letter of recommendation, I would be honored. I have taken the liberty of enclosing all the related documents, such as the recommendation forms, program materials, and [additional documents].
I would be more than happy to discuss your letter and answer any questions you may have.