Make sure you have a properly installed sissy bar before even thinking about putting any luggage onto the bar. More importantly, never have a poorly installed sissy bar if you plan on allowing a passenger to use it. Once you have ensured your sissy bar is safely installed... now for the fun part. Installing a new set of sissy bar bags!
If you're considering a larger bag, you might want to still consider having a luggage rack to support the sissy bar bag. This will add to the stability you need while riding. The last thing you want to have to happen is a major shift in weight on a bumpy road. But if you are not going to have a passenger on your seat you can rest the sissy bar bag on it and that should suffice. Smaller bags fit most sissy bars or backrests without much or an issue and attaching them can be pretty easy.
Typically you can attach a larger motorcycle sissy bar bag simply by following the provided instructions that came with the luggage. Some have a pocket of sorts that slides easily on the sissy bar while others have a strap/tie-on system. Both of these styles of mounting systems often include adjustable strong Velcro straps and/or quick-release buckles.