I issued warning letters to my maid before. The warning letter should better start with :
To : XXXX (your maid's name)
From : XXXX (the employer's name)
(XXXXXX)--contents of disapproval and ask her not to do next time.
Signed ---------------
(the employer)
Date
Acknowledge recipt by __________
(maid's name)
Date
Duplicate copy and one signed copy for you and one for your maid.
The acknowlegement docket should be signed by the maid to ensure that she will not deny that she never receive it later on.
However, by my experience, when you issued first warning letter to them, they would behave for a little while but their bad habits will return again. There is no point of continue issuing them warning letters if you find the situation does not improve. If that is the case, you better consider your position again ie tolerate with them until the contract ends or fire them for a new one.