I recently learned that if you are asked if you've, say, been to Japan, you can use "nara" and reply with "I've been to China, but I have not been to Japan" to sort of lesson the blow of your reply. What if you were sick and someone asked you if you took medicine, how would you use a "nara"-like word to say something like, "I slept early, but I did not take medicine." I used being sick since it's just a more convenient situation, but it can be anything. How would you say I have done this, but I have not done what you asked if I have done in the style of a "nara" sentence?
Unless you are already pretty fluent, my true recommendation would be to not use 「なら」 in saying "I slept early, but I did not take medicine." If a Japanese-speaker (me) had to think for a few seconds about how to say something, it would usually not be the most natural-sounding sentence.
「なら」 works better with two nouns with only one verb (as your China-Japan sentence) than with two different verb phrases.
If you absolutely had to use 「なら」for some reason, you could say:
It is the contrastive 「は」 in the last half; Do not use 「を」 if you want to sound natural.
Finally, if you did not have to use 「なら」, you could say it much simpler as:
That is how I recommend you phrase it using a nice pair of contrastive 「は's」.