should honestly be very easy. you'll need to get dual-fillament signals and then just splice the "running light" wire for the signals in with the running light wire for the brakelight. the signal's wire should be labelled or just use either of the "hot" wires (as opposed to ground). to figure out which wire is used for the brakelight's running light, pop the seat off, turn the key on. the running light will be on, no? pull one of the hot-wires going to the brakelight and you'll find out which one's for the running light vs brakelight. splice accordingly.
only thing i'll say is be careful to not cross the wiring since it'll be live while you pull connections to isolate which of the taillight wires is "running" vs "brake". there's not an aweful lot of current to zap you but you certainly don't want to short anything by accidentally tapping the hot and ground wires together. as soon as you figure out which one's which, turn the key off to do the splicing
I have it on my HD, it was simple to install there. Been looking at them for the Monster, but legally in Pa the turn signal lights if used as brake lights must be red not amber, and I haven't been able to find red lens for the turn signals .