The first - and in most cases the only - step is to download all your transactions from c-cex and import them to Koinly. There are a couple of ways you can do this:
If you decide to use CSV files, you have to make sure to import your deposits/withdrawals and trades for ALL 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 crypto tax forms!