The key to mailing a certified letter isn't necessarily in the wording.
A certified letter means that you mail the letter using Certified Mail so that you have documentation proving when the document was mailed and when it was received by the other individual.
The US Postal Service details on mailing a certified letter can be found here:
As for the letter itself.
Be sure that it is a formal letter. Begin with the date on the upper left then leave a blank line and include a block on the left that includes the following information about the person you are sending the letter to:
Full Mailing Address
Next include a space and address the letter with a formal name.
Ms. Jane Doe,
Then begin the letter. Be sure to include the exact cause for the letter, trespassing in this case, and be sure it is understood where the property that is being trespassed on is located by including the address.
I am writing this letter in reference to the repeated trespassing of my property located at 100 S Main St in Small Town, TX during the past 6 months.
And include somewhere in the letter the fact that you have informed the police department and include any action you will take if the trespassing doesn't stop (Such as lawsuits, restraining orders, or even simply contacting the police again).
Close the letter as follows:
(sign the letter here)
100 S Main St
Small Town, TX 11111
Then keep a copy for yourself and mail the other copy to them via certified mail. Be sure to keep your mailing receipts.