You have to start another wallet that is configured to use the ETC block chain, you can use another computer or create another user account and run the wallet under than user account. When you run the official wallet from Ethereum it will ask you if you want to use/support the Fork. I have not tried this myself, I converted to BTC before the fork...I was confident things would go smoothly...and thus I missed out on the free ETC...no risk no reward I guess.