After I got the item, i can't figure out what to do with it


Well, if you use the option to hold the new item it will give you some clues.


It is essentially a spell wand with some limitations.
Generally, it requires the target to dance to be able to use it. If you hook up in bed with ...

It is not clear to me where exactly you got stuck.
But if it is after the first sex scene:

After drinking the potion that changes player size, go near the bed and use the "investigate option".
It will tell you there may be something below it.
Then after going back to normal size, do it again and ...