Registering DLL files in Windows 64 bit

If the DLL is 32 bit:

Copy the DLL to C:\Windows\SysWoW64\
In an elevated command prompt: %windir%\SysWoW64\regsvr32.exe %windir%\SysWoW64\namedll.dll

if the DLL is 64 bit:

Copy the DLL to C:\Windows\System32\
In an elevated command prompt: %windir%\System32\regsvr32.exe %windir%\System32\namedll.dll

It actually looks to be the  wrong way round, but that’s the way it works.
On a 64-bit version of a Windows operating system, there are two versions of the Regsv32.exe file:
The 64-bit version is %systemroot%\System32\regsvr32.exe.
The 32-bit version is %systemroot%\SysWoW64\regsvr32.exe.