I'd like to answer you in a serious manner as you did say you are very inexperienced; if it screws up somebody's jerk off session, so be it.
1st) You two must have a clear understanding how she feels about it. It's ok if she goes back and forth with it, hesitant, etc. but if she is dead set against it never allow ANYONE to think it's okay to try anything.
2nd) If you decide you're going to do it beforehand, it's as simple as an ******* while showering. In all honesty I've gotten 'dirty' with partners, ******* happens, nothing soap and warm water can't handle. Practice not being embarrassed about a natural bodily function; again, ******* happens.
3rd) Lube, lube, lube! The more you use, the easier it will be. The 'pros' will tell you the sex store brands, I say any Super WalMart, baking section, a jar of organic/virgin coconut oil. A 16oz. jar will last months (keep your hands clean) and only a few dollars. When you get it, just stick a finger in then rub a couple of fingers together and you will know. If kept in a warm place it will liquify.
4th) It would be beneficial to learn to relax with a small toy or a finger and build up gradually...do be sure you can keep a hold of whatever you use....people lose stuff up there all the time (I lost a cock ring one time...).
5th) Until anal is your thing, do NOT do it with the first erection. The rigidity can be painful at times. Subsequent erections are more flexible.
When done right it is truly magical!
