From the Overview page, you may the click on the Transact tab at the top of your page. A prominent Trade screen with tabs for Stocks & ETFs, Mutual Funds or Options order entry pages will display. If you have multiple brokerage accounts, you will be prompted to select the appropriate account from the drop-down menu. You may then fill in the necessary fields to complete the desired trade. For helpful tips, you can click on the field descriptions.
Alternatively, you may also use the Quick Trade option on the Overview page within the Portfolio tab, or click the trade button when displayed on any page to begin the process of entering a brokerage trade.