You can do this on Poloniex.com also.

1. Log into your Poloniex account.
2. Go to Balances | Deposits & Withdrawals
3. Select SBD (steem dollars) as the coin then deposit.
4. You'll get a memo field deposit code and who to send the steem dollars to (poloniex)
   
5. Go to your wallet in steemit.com
6. Click the drop down list next to steem dollars and choose Transfer
   
7. Put poloniex into the To field, the memo code you got from Poloniex into the Memo field and how manu steem dollars you want to transfer into the Amount field.
   
8. Click the Transfer button.

You can convert your Steem Dollars to BTC directly once the SBD are on Poloniex.

Done