To couple, drive one piece of rolling stock into another just like you would on the layout. You’ll hear a distinct sound to confirm to coupling (you can turn sounds off in View / Preferences / Sounds). At that point, the cars to which you have coupled become part of the consist and you can drive away with them in train.

To uncouple, point your cursor to the junction of the two cars you want to uncouple, or to either car near the desired end, and click the left mouse button. You’ll hear an uncoupling sound, and, if you’re moving, the cars you cut from the consist will be left in position on the track. If you are stopped, you can drive away after uncoupling, leaving the cars behind.