Must be on Company letter head, must be detailed your complete name with role - designation, company's address , website, email fax - phone etc should be on letter head, must be signed by your manager or superior i.e. may be director, CEO etc.
should not fake, some companies letter head is thick with companies details printed as watermark on paper , some companies letter head have security kind of text in paper, my company issued me on letter head on letter head companies address , website email fax phone and companies registration number also there, when my manager signed his name and his designation available on sign part, and my company letter head have some images logo of company in watermark in paper with company's registration number and its not visible when you look as normal paper but when you look in light its visible some watermark. its security because my company is big corporation.
so, main point its should not be fake and atleast the signed person have to know you that you work/ed with him in past/present, becasue VO/CHC will call for varification.
OMM