The first step is to download all your transactions from Mercatox and import it to Koinly. There are a couple of ways you can do that:
If you want to use CSV files, you must make sure to import your deposits/withdrawals and trades for ALL years and not just the current one!
That's it. The only other thing to keep in mind is to tag your airdrops/forks so they appear as Income on your final crypto tax documents!