1) Navigate to Pro Trade by clicking the tab at the top of your screen
2) Under the "New Order" section of the Pro Trade interface, make sure "Limit Order" is selected, and not "Market Order"
3) Type in your ask price, or select one from the order book.
4) Click "Sell"
5) Check that your order was filled and completed in "My Orders"
Note: If you order was partially filled, you can cancel it or wait until it is able to be filled completely. If your order was cancelled, you will need to repeat the process.
See the tutorial video below for a step by step