The first - and in most cases, the only - step is to download all your data from MtGox and upload it to Koinly. There are a few ways you can do this:
If you want to import CSV files, you need to make sure to import your deposits/withdrawals and trades for ALL years - not just the current one!
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 documents!