Hi @eansteenkamp, the quickest way for you to do this would be with a service such as blocktrades.us (it was down for me yesterday) or changelly.com.

Just select the currency you want to put in (Steem/SBD) and your desired currency for exchange and enter your wallet id for your Luno account. You then send the Steem and wait for the transaction to complete. Within about 30 mins (bitcoin) it should appear in your exchange wallet.

Hope this helps.