Cancelling National Health Insurance

What's the correct way to cancel my health insurance?

I pay monthly direct from the bank.

If I plan to leave Japan (permanently) in, say, March, do I make that my final payment (or does Feb payment cover that?), then cancel?

Also, in fact, can I just cancel anytime, say, 1-2 months before a March leaving date?

How does it all work?

You can't leave or cancel your national health insurance while you live in JP.
If you will leave JP, permanently or for more than one year, you can visit your local city office, national insurance & pension funds section and fill a form that you are leaving JP on an exact day(eg, 1st Jan 2017), you can do process of your insurance card will be invalid from 2nd Jan 2017.
You must join either national health insurance or private health insurance at your work while you live in JP. This is compulsory for both Japanese natives and international visitors.