Hi, danne!
If you start a draft of the letter here, without putting any contact information in it and making the names and places fictitious, I'm sure that there will be those of us who'll be willing to come along and suggest to you how you might make improvements to it.
When I write letters like that, I like to start them with something like the following:
Dear (Name),
Please accept this correspondence as a recommendation for (maybe, first middle and last name of the candidate here) to attend the (name of seminary school). This is a task that I take seriously and hope to fulfill responsibly.
In the letter, you might also want to put in what capacity you have known the person and for how long. Why you think the person is a good, Christian candidate and how you have been witness to the person being an example of leading a Christian life as well as being a good witness for Christ.
Those are some initial thoughts, anyway...
Thanks!