I have never tried selling my STEEM tokens yet. I do have some idea on how it might be done though. First part should be easy as I see it. Simply go to your Wallet. There should be a button there called "Buy or Sell STEEM". Something like that... Click on it and it will redirect you to a digital currency exchange service where you should be able to sell your Steem Dollars for BTC or ETH or whatever digital currency you like. After that selling BTC should not be a problem. Simply google it and choose the option you like the most. Hope it helps :)