Click on the Horizontal Price Line button , go to the chart, click and drag it, as you move the mouse you will see the value for that line position, release the mouse and the line will remain on the chart. You can move any line to another place on the chart by clicking and dragging it up and down the chart.
To erase a line right click on the line and select delete. To delete all the horizontal Price lines right click and select delete all. To change the color of the lines right click on the line and select color. To link the lines to another chart of the same symbol right click on any line and select link.