Tools - Marking Cells - Mark Selected Cells

Ribbon bar. Select the Tools tab. Click Marked Cells and from the menu, select Mark Selected Cells...

Classic menus. From the Tools - Marking Cells submenu, select Mark Selected Cells (or click the toolbar button located on the Tools toolbar)...

...to add the selected cells to the set of marked cells (i.e., the Marked Cells spreadsheet layout will be applied to the selected cells) in the spreadsheet. Any previously marked cells will remain marked. See also: Format - Overview, Spreadsheet Layouts - Overview.