This kind of letter is written in the general letter format:
Recipient's name (if applicable)
Subject (Re: Jane Doe, caregiver)
Once your header is in place, you want to fill in your content. First state the intention of the letter (I am writing this letter on behalf of my former caregiver, Jane Doe, in regards to her employment with me).
Then give all the good points that you have regarding the person (Miss Doe was an empathetic, and patient women, who was always on time, and has not missed a single day while employed with me...)
Follow up with a recommendation (I believe Miss Doe to be professional and skilled at her job, and fully recommend her..)
Thank the recipient for taking the time to read your referral.
End it with your name (and title, if applicable), your signature, and the date.
Just put your complete address, name, reference and the history of your work. Do not forget the time and date.