The first step is to download all your transaction data from Decred and upload it to Koinly. There are a couple of ways you can do this:
Note that if you decide to import CSV files, you have to make sure to import your deposits/withdrawals and trades for ALL years !
That's all you have to do. The only other thing to be vary of is to tag your airdrops/forks so they appear as Income on your generated crypto tax forms!