There is no standard format/template for such letters. You may narrate the reason for termination in details, substantiate with proof for poor performance and then conclude as
"As per the terms and conditions of your Appointment Order you would be paid one month Gross salary as Notice pay"
Alternatively you can ask employee to submit resignation and then relieve him with one month pay, provided if this is ok with your employer.
23rd April 2019 From India, Madras