The first - and in most cases the only - step is to download all your transaction data from OTCBTC and upload it into Koinly. There are a few ways you can do that:
Note that if you want to use CSV files, you have to make sure to import your deposits/withdrawals and trades for ALL years !
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 forms!