Make sure you have Microsoft .NET Framework 4 ( Available here or via Windows Update )
Windows 7 and Windows 10 have this already installed.
2a. Pre-requisities with FSX, FSX-SE, P3D
Download and install FSUIPC library from Pete Dowson
If you have multiple versions of FSX (e.g. both FSX and FSX Steam), FSUIPC installer will ask which version you want to install it to.
Free unregistered version is sufficient, you don't have to buy it (but it is encouraged if you wish to use advanced features).
2b. Pre-requisities with X-Plane on Windows
Download XPUIPC (version 2.0.0.0) from the link here.
Install the plugin according to its readme instructions.
Start X-Plane and verify that XPUIPC item is available in Plugins menu.
3. Install RemoteFlight Server
Download and install RemoteFlight Server, which is available here. Once installed, you will find the server in "Inputwish > RemoteFlight" directory in Start Menu. The application will check and download updates automatically.
Alternatively, you can download a standalone version of RemoteFlight Server, which doesn't need installer. If you download this version, all you need to to is to extract the ENTIRE zip archive into a folder (e.g. on your desktop).