Use a compass. You can get borrow one or purchase one a WalMart for about $7. This is the
compass I have. The compass on your smartphone is not overly accurate.
An alternate is to re-do your plot at
this link, then switch to "Satellite" view (click on "Satellite" at top right), zoom all the way in, then click on "Show lines pointing to each transmitter". You can the lines from your location to the towers to pick out landmarks that can assist in pointing the antennas. It's not quite as accurate, but then you can move the antenna a bit to the left and/or right until you find the sweet spot.