The first step is to download all your transactions from Bitstamp and upload it into Koinly. There are a couple of ways you can do that:
If you want to import CSV files, you need to make sure to import your deposits/withdrawals and trades for ALL previous years - 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 generated crypto tax documents!