If you have already some steem dollars and want to turn them into bitcoin all you have to do is to register in : https://bittrex.com
Then you have to go to wallet and in the search type STEEM, you will find there both Steem and Steem Dollar wallets :
Choose one of them for example Steem Dollar and click on + and you will have the New Address button, click on it and a special memo and account will be generated for you :
Then go to your wallet in steemit and transfer what ever you want into bittrex :
After recieving Steem or Steem Dollar in Bittrix, you have to go to wallets again and choose Steem Dollar for example :
then go down and choose sell Steem Dollars :
Select better price down there in ask and it will apear in sell Steem Dollar above :
Then just click Sell Steem Dollar button and you will have your money in bitcoin and then it's easy to withdraw them from bittrex and send them into any wallet you want ☺
Follow @Clixmoney for more useful tutorials ☺