If you are an american citizen or a permanent resident and you have income more than the amout needed by the poverty guideline for the total number of persons in your family and the family of the person you are sponsoring, download the form 1-134 and follow instructions in that form. That form is actually sponsor letter for the DV winner. If you are not either a citizer or perm resident then you can not fill up that form. In that case you can just write a plain letter mentioning that you are ready to provide the required accomodation for the person or family until they get job. Send your proof of income, address and phone number as well. Hope this helps.