Dear,
Please check the suggestion below best to my knowledge. I hope it helps.
You can specify in your offer letter
"We would like to inform you that you have been selected for a (for example sales and marketing trainee position/Intern in HR Department) in our company for (duration-for example six months/one year). During the training period, you will be required to undertake all the duties and responsibilities assigned to you by your supervisor.
You will be paid (STIPEND) per month/year as a (POSITION) and shall be subject to review upon completing the training period."
You shall not be eligible for any other monetary benefits during this period. (this clause can vary company to company)
Apart from this mention the basic details like:
Date
Address
Subject
Terms and conditions as per your company which you want your trainees to follow
Start date
Hours of work/timings
Holidays (if applicable)