How does one go about turning that on in the TA?
Technicality, it's on you enable it for your plane.
It's slightly different for friendly or enemy planes. For friendly, hit <Ctrl><Tab> to enable friendly lock. Then with a friend in your like of sight, hit the <TAB> key to highlight that target. It puts a rectangle around the gameID and brings up a small green + for your guns, showing impact point of your rounds. Depending on your gun platforms armament you may and usually do have more than one green +. I believe it's <Shift><TAB> to cycle through targets in your field of view.
For enemy it's almost identical but you do not have to 'Enable Friendly Lock' with <Ctrl><Tab>. To unlock a target look away with no other targets insight and hit the <Tab> key.
It's not perfect and you may need to lead the + a little at times but it's a nice tool to show people without much flight sim experience just what lead is and why you need it to hit things and get kills.
