To make it a complete sentence, you need to add a verb and a subject. For example:
* "In the letter, she explained her reasons for leaving."
* "In the letter, there was a detailed account of the incident."
It's also important to consider the context. If you're referring to something within the letter, it's more natural to say:
* "In the letter, she wrote that she was happy."
Or, if you're referring to something attached to the letter:
* "In the letter, she enclosed a copy of her resume."
Let me know if you'd like help with a specific sentence!