The best way I can explain it (with an easy visual) is that the lid hinges work like door hinges in your house. On most hinges (including your doors and lids) there are two halves that rotate around a pin.
The object of the dental floss is to wrap the pins and fill the gaps between your hinge halves . Filling the gaps adds tension to the hinges which keeps them from moving up/down/forward/backward.
Start with a 12-14" long piece of floss and stick it between the hinge halves below the pin from the inside of your bags. Once you see it come through the outside pull it through (leaving a 2-3" tail) then push it back through over the top of the pin. Repeat this process at least 5-6 times per hinge section and it will eliminate rattle. Once you finish each section tie the 2 end of the floss together and snip the excess.
Note that it is better to have too many wraps than not enough. With too many you'll have more tension on the hinges but the floss will last longer.
Retired AD USAF, IYAAYAS!
2008 V-Star 1100 Custom Bobber project
2011 Crimson XR with Forged Highway Bars / Hard Bags / Passenger Backrest / Tall OEM Windshield / OEM Highway Bar Closeouts / Hardbag Protection Bars. Ness goodies including Smooth Layback Plate Frame/Mount, black Rad 3 Mirrors, black deep cut grips, black deep cut drivers floorboards, black deep cut shifter/brake pegs, and black deep cut passenger pegs. Witchdoctor black rack.