You don't need an account, but if you want you can make one by clicking on "Sign up" - this will keep a log of your transactions.
I'm OCD so prefer to have an account where I can check my old transactions.
Select the currencies you wish to exchange. Under "send" on the left is the currency that you have, on the right under "receive" is the currency that you want to exchange to (and hence receive). Click to select the currencies you want:
I am converting Steem to Bitcoin which will look like this:
Enter your bitcoin address at the bottom and click "Get Deposit Address". The next screen will come up:
The Steem Address will always be the same "blocktrades" but you need to also use the Memo Address as shown. Make sure you don't mix these two up.
Go to your Steemit Wallet by going to your blog and clicking the Wallet button as shown:
On your wallet page click the disclosure triangle next to Steem or Steem Dollars.
I am converting Steem so will click on Steem.
Click on Transfer. The following window will come up:
Enter the address as "blocktrades" and the memo you received earlier in the appropriate box. Be careful not to mix them up, although in my experience if you do the wallet will not allow you to make the transfer. Double check everything before clicking on Submit:
The transaction will show up at the bottom of your wallet page in the History section.
It will also show up on the Blocktrades page - I'm not sure why but it is showing as the second transaction on my account page but normally it will be at the top.
Finally check your Bitcoin (or other receive currency) wallet and after the appropriate amount of time your coins should be there.