It is not a simple matter of just writing a letter.
You should retain an experienced immigration lawyer to review all the facts and advise you accordingly.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
When you are dealing with an I-601 waiver (or similar) and need to write a hardship letter, it is important to remember that it is not actually a "letter." It is actually supposed to be a whole story that is tailored to bring out certain characteristics of your relationship. It needs to be very well crafted in order to be effective.
You can find some information here: www.swagatusa.com/educational-materials but I strongly advise that you do not go at this without any assistance.
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An extreme hardship letter states the hardship, explains the effect the hardhsip will have on the USC or PR and evidence should be attached.
Consult with an experienced immigration attorney to assist you.