Whats a Good DLL Injection Tutorial?

Hey everyone! I'm trying to write a custom DLL injector. I've found forum posts that have the source code and videos walking through writing the code, but none of the posts or videos I found actually explain what the code does. I was wondering if anyone knew of a good C++ tutorial (written or video) that describes the code in detail. If anyone can link me to a good resource, it would be greatly appreciated.