The article by Robert Green, CPA, is excellent. It's the go-to resource on this topic. Interesting that he states that the cost basis per Bitcoin Cash is 9.5% of the original value of the Bitcoin, and for some of us, we had to buy hardware wallets to even be able to get the Bitcoin Cash, so there is an added expense, that may or may not be added to this cost basis. I was able to confirm the $266 figure he cites using historical records of Bitcoin Cash prices, and since it's like "found money" or prize winnings, regrettably it has to be reported, as does the subsequent sale of BCC later on, as CPA Green explains. So, if you did like someone I know, and dumped your Bitcoin Cash a few months later when the price spiked up, that sale is a separate entry and also taxable. Still, it's all good, because you got a bonus with this hard fork that some speculated would be worth mere pennies a week beforehand.
What is grievous is the way the exchanges, including Coinbase and others, pretend these hard forks never happened and keep the proceeds to theirselves. (Coinbase did correct that for Bitcoin Cash, but has not done so for Bitcoin Gold.)
I have read another article, also by a tax firm, that says that stated you declared the proceeds from the Bitcoin Cash in the year you receive them, but for practically everyone, that is FY 2017. Bitcoin Gold may be a different story.
It is easy to enter this into Form 8949. TurboTax has done a nice job of making this section work quite well and there are a bunch of tips on here on how to do this.
