Process injection is a very popular method to hide malicious behavior of code and are heavily used by malware authors.
There are several techniques, which are commonly used: DLL injection, process replacement (a.k.a process hollowing), hook injection and APC injection.
Most of them use same Windows API functions: OpenProcess, VirtualAllocEx, WriteProcessMemory, for detailed information about those functions, use MSDN.
InjectProc uses SetWindowsHookEx function, you can try different ways to installing hooks, for example, EasyHook
Windows x64 binary - x64 bit DEMO
vc_redist.x64 - Microsoft Visual C++ Redistributable
InjectProc DEMO - Process Injection Techniques
Works on Windows 10 build 1703, 64bit.
I've not enough time to test other systems and make it portable if you have enough time please contribute.
I create this project for me to better understand how process injection works and I think it will be helpful for many beginner malware analysts too.