You might be wondering why girls oppose anal sex a lot. Some of them might have tried it once with the wrong guy and it was so painful that they decided that they would never do it again. Still, there are some ways to make it pleasant for her, too.
What you should keep in mind is that there is no natural lubrication in the anus, so when even a finger slips in there, it can feel like sandpaper forcing its way in. So, either if you use your saliva or body lotion or anything slippery, it can help a lot in the beginning. The best way to do it is to use your tongue to make those muscles relax and to lubricate at the same time, so your fingers can more easily find their way inside. But while saliva can be enough when only one or two fingers are inserted, the actual penetration needs something stronger than that.
Ideally, silicone-based lubricants should be used, as they are long-lasting and slippery, but when you use a condom, then water-based solutions are suggested, as they do not react with the rubber. If you are out of everything, but you do not want to miss the opportunity, though, then you can still use our body lotion, but you should not do it frequently.
As for the penetration itself, please keep in mind that those muscles do not usually open wide, so you need to take your time and let them adapt to your girth. Once it is done, you will feel it, since they will become slightly looser and that is when you can insert your penis more deeply inside. When your full length is in there, keep still, until your partner makes the first move. After this, just enjoy!