First take the pain killers. Then drink lots and lots of alcohol. Lastly, apply the anal lube when she is starting to go into a slight coma and I promise she won't feel a thing... Jesus where are kids learning sex education now days... wait... your 20 and not a kid so thats even worse...
No seriously, go to a drugstore, wally world, Walgreens, whatever, and get some silicone based lube and a enema (might want to get them at seperate times if you get embarrassed lol. IF YOU DON'T GET EMBARRASSED, run to the pets area and pick up a can of cat food. That will have the checkout person really eyeing you then lol.
Silicone based lube is the best for anal sex or play (unless your using a silicone toy but then you can just put a condom on the toy). Waterbased lubes can cause internal discomfort if you are allergic to the ingeds (whatever that sugar crap is called gly something). To test it just put a glob of it on your finger and stick it up your hole. What 5-10 secs. If it burns like hell then you are probably allergic to it. Just relax and it will stop in a few mins. If you can't take it, just go sit on the pot.
Anyways, use the enema a few times and flush her out good unless you are into playing with poo (hey some people are lol). Tidy up. Use plenty of lube and mess around. Just go nice and slow with the playing so she can get used to the new feelings and learns to relax her ass. It will probably take a couple of times and days before you can do any major GNP on her ass.
Have FUN!
P.S. Oh BTW, if she stuck a marker up her ass halfway without any lube AND enjoyed it, it ain't the first thing and first time she's put something in her ass