Most Helpful Guy
Easier if she is already aroused. Don;t go straight for her butt; spend time in foreplay and let her have an orgasm or two from oral, fingering, vibrator.
Move on to anal play. Tease her with some touching without penetration. If you are adventurous, move on to rimming while you finger her clit.
Then, put some lube on her butt hole and start penetrating her - slowly - with one finger. Go to fast and she will tense up; go slow enough and she will relax. When she is comfortable with one finger, move on to two fingers or a vibrator.
When she is comfortable with that, it is time for penetration. Use lots of lube and go slow. As soon as you feel yourself moving past her sphincter, stop for a moment and allow her to get accustomed to how it feels. Then start advancing slowly until you are in as far as you can go. Then start thrusting slowly until she is ready for more action, then she will probably beg you to fuck her ass until you cum.
It might also help if she fingers her clit while you are fucking her ass or if she puts a vibrator in her pussy (but don't put anything in her pussy that has already been in her ass or she will get a UTI.)