1. Open the Word document and switch to the Insert tab. Locate and click the Title button. This will automatically switch to the header editing mode, and the header area will be displayed in the header section of the document page.

2. Select the Design tab. Locate and click the Margins button in the Header and Footer Tools group. Select a custom margin option.

3. In the Custom Margin dialog box, locate the header section and set the top and bottom margins to 0. And confirm and close the Custom Margin dialog box.

4. In the title area, position the cursor to the position of the text and enter the title text to be displayed.

5. On the Design tab, in the Header and Footer Tools group, find the Horizontal Line button.

6. In the drop-down menu, select an appropriate horizontal line style, such as "horizontal line 1" or "horizontal line 2". The horizontal line will be close to the header text.