Regarding the Sportmans guide tarp

http://www.sportsmansguide.com/cb/cb.asp?a=97247

After trying to get it set up was wondering how this tarp is oriented, because there are no tieouts in the midline so it can't easily be hung rectangular, only at a diagonal. Do you have to use a rigdgeline to get it rectangular? If so, how would you secure it on the ridgeline so that it doesnt slip?