Choose View - Styles to open the Styles deck in the Sidebar.
Click the icon at the top of the Styles deck for the style category of the new style.
Click in the document where you want to copy the style from, for example, in a paragraph to which you applied manual formatting.
Click the Style actions menu icon at the top right corner of the Styles deck and choose New Style from Selection from the submenu.
Enter a name for the new style in the New Style from Selection dialogue box.
Click OK.
Styles actions menu icon
Choose View - Styles to open the Styles deck in the Sidebar.
Click the icon at the top of the Styles deck for the style category of the new style.
For paragraph, character and list styles, select at least one character in the style that you want to copy, then drag the selection to the Styles deck and release. For frame styles, select the frame and hold the mouse button pressed until the mouse icon changes, then drag to the Styles deck and release.
Alternatively, you can drag-and-drop the selection onto the respective icon at the top of the Styles deck. You do not need to open that style category in advance.
For page and table styles, you must use the Styles actions menu at the top right corner of the Styles deck. The drag-and-drop method is not possible with these styles.