How does someone train using 了 correctly?
It is a tough question I would give it a try...

了
(1) [aspect marker] indicating completed action
(2) [final particle] indicating change of situation

Let's use a shorter sentence.

I go to see the flower (我去看花)


If you want to state the verb 去 (go) is complete, the sentence would be (我去了看花)

If you want to state the verb 看 (see) is complete, the sentence would be &quot;我看了花&quot; (I've seen the flower)

if you want to indicate the verb phrase 去看花 (go to see flower) are completed action the sentence would be &quot;我去看花了&quot; (I've gone to see the flower).

if you want to indicate the situation has changed, the sentence would also be &quot;我去看花了&quot;. You would say that if the context is Q: &quot;去看花沒? A: &quot;我去看花了&quot; (situation changed from 'haven't gone to see the flower' to 'have gone to see flower')

if you want to emphasize the action &quot;去看花&quot; then sentence would again, be  &quot;我去看花了&quot;. You would say that if the context is : &quot;老闆，工作做完(現在)我去看花了&quot; (Hey, boss, the job is done, I am going to see the flower now)


我去看花了 can has three different meanings in different context, deciding where to put 了 in a sentence without other context is not easy indeed.
Conclusion:
To use 了 correctly, you have to:

Identify which action is completed and which situation has changed, just think about what your sentence would be like in English, and you will find some clues.

Consider what you want to express by filling in context in your mind, like in my &quot;我去看花&quot; example.