How do monetize my Youtube videos?

A friend of mine created over 100 videos on Youtube about haircare and she wasn't aware that she could generate income from her videos. I told her that she could monetize her site with adsense but every time that she applies for adsense, her channel keeps getting rejected. I logged into her account to try to monetize it and it says that her videos are now being monetized but she's still unable to log into adsense. Can anyone walk me through the steps to do this properly?