Would like to install crash bars on a virago xv250. How do I do this? Thanks.

Installing the crash bars depends on the brand you purchased. They usually come with instructions.
In any case, crash bars are attached to the upper section of the frame, on each side of the foot-pegs and under the main bolts of the engine block.
The crash-bars should stay rigid, without any vibration nor limit access to the foot/hand contols.