How to Attack
If you’re in the tutorial, you need to queue up two attacks. It’s teaching you that you can fire more than once in a turn.
In regards to the process for targeting itself, here’s an overview from another thread on the subject.
As an overview, first select the attack action, then on the timeline move your mouse to position when you want to fire. This should update the prediction for you automatically.
Click to place the action on the timeline.
Next, it should ask you to pick a target, move the cursor around on the map to target the unit you want to fire at. You can also hold ctrl if you want to fire at a building or at the ground.
After that, if you want to adjust the timing of the attacks, you can left click and drag them on the timeline. This will help you avoid overheating.