your job will be to erase all doubt from her mind. like i said, make her completely comfortable in her sexuality, it's tricky like that because if they're even slightly distracted it won't work. and the thought of taking a leak on their partner is horrific enough to be considered a distraction
there's dozens of articles and sites you can show her if she needs reinforcement or "proof" that it's normal (and probably the greatest thing she'll ever experience in this lifetime).