If she will kneel with her butt in the air, and her breasts on the bed or rug, then you enter her from behind. Penetration is deepest in this position (along with her sitting on you while you lie on your back) so that your semen is discharged directly onto her cervix. Stay inside her for a few minutes to make sure all your semen has gone into her and to prevent leakage.
When you pull out, have her lie down with a pillow under her lower belly to keep her vagina tilted downwards. This helps the semen stay in her and pooled on her cervix.
Don't forget to give her a back massage after you finish while your penis is still inside her. Also you can reach her breasts to fondle them.
Mounting her from behind, if you are at the right angle, causes her corpus spongiosum to be massaged. This is the same as yours; only her urethra goes about straight down and so does the CS, while yours goes along the underside of your penis for several inches. Both the male and female CSs are very sensitive near the opening of the urethra - in the female the most sensitive area is the grafenberg spot. It is against the outside of the vaginal canal just in front (or, when she is kneeling, just below) of her vagina, just back (or above) the opening of her urethra.
You may also be able to reach her clitoris. Gently stroke and massage it and the hood (corollary to your foreskin). Get her to tell you just what to do. It is very sensitive and the sensitivity varies a lot at different stages of her excitement.
The clitoris is commonly thought to be only an inch or so long when erect. However, it runs all the way back to just in front of her anus, just like your penis does. The clitoris splits and passes on either side of her vagina, then joins again above her perineum. If you are careful you can feel its swollen, rigid form just inside her vulva.
If you can help her orgsm while you are inside her, she will be eager to do it this way again. Soon.