Building a liquid-propellant rocket engine is pretty much the pinnacle of amateur engineering projects.

The 1967 book How to Design, Build and Test Small Liquid-Fuel Rocket Engines can be read online, or you can download the entire file in zip format.

From the introduction:

The purpose of this publication is to provide the serious amateur builder with design information, fabrication procedures, test equipment requirements, and safe operating procedures for small liquid-fuel rocket engines.

Also, don’t forget the bible: