1. Click the Start button on the toolbar at the top of word, and then click the underlined icon button under the letter "u":
2. Place the cursor where you want to underline, and then press the space bar on the keyboard, and the underline will come out. The length of the underline can be controlled according to the number of spaces:
3. Click the Insert button on the top toolbar, then click the Shape button, and click the straight line shape icon in the expanded shape options:
4. Hold down the "Shift" key on the keyboard, then hold down the left mouse button and drag from left to right, and a horizontal straight line will be drawn: