2. Relaxing the muscles. Start by slowly sliding 1 well lubed finger inside her anus, gently! Not too deep to start with. Her body language will guide you. When she's sufficiently relaxed and comfortable with you fucking her ass with 1 finger then gently slide a second finger in and repeat the relaxing gentle finger fucking. Often times, paying attention to her clit will assist in her comfort and relaxation. You have two hands and a tongue, you can easily use your other hand to rub her clit, or better yet, gently lick and suck her clit as you slide your finger into her ass.
3. LUBE! LUBE! LUBE! This simply can't be stressed enough. The right lube is essential and it may take time to find the one that works best for her. It's always recommended that you use a water based lube, but they can wear out easily. Some of the new products can make her more or less sensitive, and some even tingle, which she may not appreciate. Talk about it before, during, and after. Find what works for you both. It's not just about you, it's about her pleasure too.
4. When she's ready for you to slide your cock in her ass, get her into a comfortable position. For someone new to anal sex, you may try the "Spooning" position. It allows her a little more control of the depth and stops YOU from going too deep. LUBE your cock just like you did your fingers. Once it's sufficiently lubed, slowly slide the tip in, a little at a time, let her breathing and body language guide you. Ladies, feel free to place your hand behind you against his stomach and hold him back until you are ready for him to go further inside you. Men, remember if she does this, FOLLOW her hand, if you feel pressure against your stomach from her hand it means STOP for a moment, let her body relax again.