Firstly, you need to unlock the ability to build an Orbital Relay - This can be done using the Society Research technology tree (Tier 4 technology - 'Orbital Relay Development'). Once you have this, you can construct it in one of your cities districts. For anyone that's played any previous Age of Wonder games, this is basically the same as constructing a Teleportation Gate, allowing your troops to travel between two Orbital Relays. Send a unit / army from one to another to trigger the achievement. Armies will take Orbital Relay routes in to consideration when you set them waypoints. In all likelihood, you will unlock this playing the game naturally.