The Global Support webpage contains detailed information about paying foreign nationals in the United States. Links to the relevant pages are provided below. 

Overview of Payments to Foreign Nationals - general information for making payments to foreign nationals and FAQs 
Honorarium Payments to Foreign Nationals - how to pay an honorarium 
Forms Needed to Process Payments - related forms
UW Tax responsibilities - what is required from the University
Tax Responsibilities of Foreign Nationals - what is required from the foreign national
Glossary of Tax and Immigration Terms - terms you need to know
Payment Flowcharts & Checklists - miscellaneous charts and lists
Description of Forms - form number, form name and purpose of the form
Non-Resident Alien Payment - related information

NOTE: If you plan to provide services or do business in a foreign country, particularly Canada, please contact the tax office (taxofc@uw.edu).