In your Steemit.com wallet, there is a drop down next to your Steem total. Select the Sell option and it redirects you to https://blocktrades.us. You specify the amount you want to sell. Blocktrades provides a transaction ID. You then transfer Steem to @blocktrades and put the transaction ID provided by Blocktrades in the steemit transfer memo field. Bitcoin is then sent to your wallet instantly.