If you want to make your own injector, follow these basic steps:
1. Get the address of LoadLibary in your own process (GetModuleHandle, GetProcAddress), it is the same in every process.
2. Get a handle to the process with all rights (OpenProcess)
3. Allocate some memory with the size of your dll path (VirtualAllocEx)
4. Write the path of your dll to the allocated space (WriteProcessMemory)
5. CreateRemoteThread with the start address of LoadLibary (Step 1), with the arguments being the address of the memory you allocated.(Step 3)
I hope this helps.