If you want to insert a watermark behind merged cells in Excel, there are a few things you need to do first.
1. Select the range of cells that you want to merge. To do this, click on the first cell in the range and then hold down the Shift key while you click on the last cell in the range.
2. On the Home tab, in the Cells group, click Merge & Center. This will merge the selected cells into one cell.