You need to state the facts as you/your mom know them.
1- date of the marriage, where they were married (county and state), where they lived together when married and for how long.
2- date of separation, and where they each have lived since then.
3- Your mom’s current legal name (this is what the notary is for- just to certify that the person who presents the letter has shown/verified that she is the actual person being named i.e. matches up with her driver’s license or other document.)
Google around and you can probably find a template. But make sure you include this information at a minimum.