Remove the drybag and leave the three clips unclipped. Then run that center strap through the top daisy chain loop and around the fork crown. Make it tight. Once the harness is tight, then add your drybag.

Also, you're going to want to put a few layers of frame protector strips, or my favorite, electrical tape anywhere the harness or straps will rub. One short ride is all it takes to rub the paint off.