Please be very very careful when not only physically spanking/whipping etc but especially when trying to leave marks even more so when those should last several days.
It can be easy enough to leave a red bum, as you can keep going until it goes red.
However, to leave marks for several days you need to tear the skin, the line between this and scaring someone can be a very fine line. She will also be a deal of pain whatever she does in these days and sitting can be an very unpleasent experience.
Mistress', Masters and Dommes have to be very careful and slowly build up as to what leaves what marks. It is not recommened for a novice to go straight into creating marks.
My suggestion would be to talk to here and make sure she is 100% certain this is what she really wants and not simply a red bum. There is a lot of difference between this and creating marks for several days. After she is sure, then I would suggest taking her to a domme of some description, one that really knows what they are doing and will not permentaly mark her or create real damage.
Please be very very careful with this, I would hate for you and/or your girlfriend to regret anything.