All your in-context editing is performed in Block Areas. These areas hold the content and functionality blocks that make up your web pages.
Just put the page in Edit Mode, mouse over a block area and click.
Click "Add Block" from the contextual menu.
Select the block you'd like to add from the list of all available types, you can filter the list by typing in the text box.
The block that you select will then present you with a form. This form allows you to personalize the block to your site and situation. When you have everything the way you want, click add and the block will appear on the page. Some blocks have additional preferences that can be changed in the dashboard.