I generally turn on "Gridlines" when I turn on "Rulers".
That will give you a decent approximation of the printable area of your word document for the placement of various components.
EDIT: Sorry I left out placing the actual lines. Just use the "Insert / Shapes" pick your line and color, and other properties, draw it on your doc and move it to where you want it..
You can move it wherever you choose
A normal document with typical margins of one inch should show a horizontal area according to the ruler of six and one half inches, so.....
Horizontally speaking three and one quarter inches would be half of that.
Likewise vertically with one inch margins, that leaves you with nine inches remaining as a printable area so, four and one half inches would be the center of that.
That's a typical eight and a half by eleven piece of paper using U.S. English measurements.
Adjust accordingly for other paper size and if you're somewhere that uses the metric system....
You're on your own.
You can always just put your lines on your document and eyeball them using the "Looks Good To Me" method.