All the details for Multiplayer connection are given in the Multiplayer Forum here.
For FSX (only) all it requires are two small programs downloaded and installed, with two short-cut Icons placed on the Desktop...http://www.chocolatesoftware.com/
FS Host Spy,
FS Host Client.
FS Host Spy shows a list of all the IP addresses available to connect to.
FS Host Client is a window to enter the IP address of your choice (both Players via PM Message, or a Forum Message).
Once completed and connected, with FSX running and positioned at the agreed (both players) location of your choice, pressing "Enter" will bring up the "Chat Window" for players to correspond with each other!
Most of the flights will take place over an agreed time and area, involving a Tour, Fly-in, Air Show, or practice flights!
Installing "Plan-G" and/or "FS Tramp" will provide a "Radar map" (similar to FS Navigator) showing the positions of all the player's aircraft in real time!
Ideal for teaching Radio Navigation, take-off+landing procedures, etc.....
...or just fun fly-arounds together!....
Paul....flying around...(or square)...
The connection for FS 2004 is very much simpler...(FS 2004 was specifically designed for Multiplayer!)
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.