The first step is to download all your data from Magnum Wallet and import them to Koinly. There are a couple of ways you can do this:
Note that if you want to use CSV files, you need to make sure to import your deposits/withdrawals and trades for ALL years and not just the current one!
That's pretty much it. The only other thing to be vary of is to tag your airdrops/forks so they appear as Income on your final crypto tax documents!