Hello
First of all, I would like to advise you against writing the letters yourself. Not only is it unethical, but when you write it yourself, your writing style will creep in these letters and match a lot to the writing style of your essays. The adcoms will see through this and this will destroy your chances of getting admission at all. I would suggest you to convince your profs and boss to write the letters themselves, or find other recommenders who would write the letters themselves. If you cant, I suggest you hand over the task to someone else.
As to what should go into your letter, university websites would have clear instructions on it. What qualities your recommenders should highlight and comment upon, etc, different universities will have different advice for it. Club all the instructions while getting your letters ready. They should be as specific as possible. For example, your letter shouldn't just say that you excel at multi-tasking, rather show it with an example, a situation where you multi-tasked effectively.
Hope this helps
All the best
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