While issuing the warning letter, it is important to quote acts of indiscipline. We need to quote what happened, where it happened, when it happened, in whose presence it happened, was there violation of any rules, regulations, SOP etc, was the employee absent, etc. Issue of warning letter on "indiscipline attitude" is vague. In the warning letter it is better to avoid vagueness.
Taking clue from the above, I recommend you writing warning letter on your own and uploading it here. Some senior like me will correct it.
19th October 2016 From India, Bangalore