The first - and in most cases the only - step is to download all your data from Blockchain Wallet and import them into Koinly. There are a few ways you can do this:
If you want to use CSV files, you must make sure to import your deposits/withdrawals and trades for ALL previous years and not just the current one!
That's pretty much it. Don't forget to tag your airdrops/forks so they appear as Income on your generated crypto tax forms!