If it came from the factory with slide locks there should be square white stickers on the outside wall showing where to place the locks. Unless yours are different, you twist the lock to expand them so that they fit snugly between the top of the slide and the wall.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)