Take a look at Form W8 and its instructions. This may be what you're looking for.
There are a variety of Forms W-8 depending on the recipient of the payments. There are several other requirements that may apply depending on your specific situation, such as having an EIN. Often foreign persons prefer not to have EINs and, in such case, there may be structuring opportunities. If the foreign person is claiming Treaty benefits, make sure you have fully scoped out the Limitation on Benefits provision of the relevant Treaty. I often see that error where people/companies assume they can claim benefits.