Hi,
I faced this question myself but couldn't find an official reply. I finally found the answer in a few comments but thought to create a post for it. After all, you don't want to "experiment" sending cryptocurrency because it can be lost :)
But it turned out, its really simple. You just send STEEM to your @username, e.g. in my case I just send it to graphicalx
So I had bought STEEM using Bitshares.com, and here's the process to transfer it:
Simply go to Deposit/Withdraw
Click on Withdraw (by default Deposit will be selected)
Select the coin you want to withdraw (OPEN.STEEM). We're using the OpenLedger service to withdraw
Now click on withdraw
You'll see your STEEM balance. Enter the amount you want to withdraw, and in address, enter your steemit username. Not your display name, but your @username, like mine is graphicalx
Note: in memo you can enter any information/reminder that you want. Basically a memo. Remember that this will be public and shown to anyone who visits your wallet section on SteemitFinally click on Withdraw. You will then be shown a confirmation box that includes the fee you'll be charged.
Tadaa! You can now view your Wallet to see the STEEM balance updated
PS, remember you might want to experiment doing this with a smaller amount to be safe