Add the new forked coin as "reward / bonus", "gift", "air drop" or "income" (depending on the tax laws of your country), .

You can also enter them with the cost base of 0 - there are tax consequences for this explained here

In the case of the BCH fork, some exchanges originally imported the two chains as BCHABC and BCHSV. Then most returned to calling BCHABC only BCH again. In this case please a) rename all BCHABC back to BCH and only enter BCHSV as a fork by entering it new or changing the transaction type from deposit to reward or other.