Select the Line button. Then your first click on the grid will start a line.
Move the cursor to where the line should end, and you will see a preview of where the line will be drawn. If you would like to start over at this point you can press ESC on your keyboard. Otherwise click to complete the line.
You can draw additional lines without having to select the Line tool again.
Make the start and the end of the line at the same place to draw a point.
Use the drop-downs to select the line color and thickness.