Curious

Have some drinks, tie her face down with pillows under her hips to forcer her butt up, eat out her pussy, lick her anus, finger her asshole, use lots of lube, put in ear plugs and go for it till you cum in her ass. Don't forget to have her suck your dick right after.

Get her to lay on her front - this relaxes the muscles. Best lube is called booty relax from Ann Summers

Lots of AstroGlide and go slow

My wife LOVES anal sex because she can feel the sensations in her pussy at the same time. Makes her cum SO HARD! Tell your girlfriend this...she will thank you for it, believe me.

You need to lube her good work a finger or two for awhile trust me that cock will fit

I've been having sex with my married fuck-buddy Mark since shortly after college. He's huge...close to 9" cut and very thick (think toilet paper tube). When we first started having sex I begged him to fuck me (I was an anal virgin) but he refused, saying he was way too big and that he'd probably hurt me or do some internal damage. Undeterred, I bought myself an "anal intruder kit" at a local adult bookstore that came with five different sized butt plugs...from a small "starter" size to the largest which was about 6" long and 3" around at the middle (they were all cone shaped). I showed Mark the set and he said when I could insert the largest one comfortably he'd fuck me up the ass. It took about six months...oftentimes wearing them every single day beneath my clothes...but when the time came it was heavenly! Mark pounded the shit out of my ass and shot ropes of cum inside my rectum.