Most countries are member of the International Civil Aviation Organization (ICAO) and as such have signed the "Chicago Convention 1944" (111 pages, 2 Mb). Aviation rules and regulations in member countries are based on the recommendations of the ICAO.
The FAA has published an advisory circular for those wanting to certify product and parts for homebuilt aircraft.
This document describes the certification procedures for products and parts, for the purpose of operating amateur-built aircraft.
Excerpt from the document:
This advisory circular (AC) provides information about Title 14, Code of Federal Regulations (14 CFR) part 21, Certification Procedures for Products and Parts, § 21.191(g) for the purpose of operating amateur-built aircraft.
This AC provides specific information and guidance to amateur aircraft builders on: Certificating and operating your amateur-built aircraft, what to do and know before building an amateur-built aircraft, designing and constructing your amateur-built aircraft, fabricating and assembling your amateur-built aircraft, registering your amateur-built aircraft.
To continue reading see the pdf below: