The first step is to download all your transaction data from NXT and import them to Koinly. There are a couple of ways you can do this:
Note that if you decide to use CSV files, you need to make sure to import your deposits/withdrawals and trades for ALL years and not just the current one!
That's all you have to do. Don't forget to tag your airdrops/forks so they appear as Income on your generated cryptocurrency tax reports!