To personalize the toolbar in Power BI Embedded for adding buttons, use the Power BI JavaScript API. When embedding the report, you could turn off the default toolbar components and configure settings to control the view of various elements, such as filters and navigation panes. Custom buttons can be put on top of the embedded report using HTML and CSS, and by using JavaScript, you can trigger specific actions like applying filters. To make the entire experience seamless, develop the interface in a way that has tooltips and loading indicators and is responsive so that the interface remains intuitive and accessible to any device.