The first - and in most cases, the only - step is to download all your transaction data from OOOBTC and upload it into Koinly. There are a couple of ways you can do this:
If you want to use CSV files, you need 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 final cryptocurrency tax forms!