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