start off slow and take your time, when her skin is really warmed up and red, it will mark without as much force (as much impact to underlying tissues). The only dangers I would know enough to warn about, are one: don't do anything that might cut off her circulation/ cause numbness, like have her hold a very specific position for a long time, that can cause deeper damage than just bruising and two: make sure she is not going to ignore her own internal "comfort meter" just to please you. If you have never played rough enough to leave marks in the past, she may be the type who doesn't mark easily and the level of roughness you are both wanting, could actually be unsafe. Probably not, but just make sure she knows she can stop any time, if she needs to.