by doing DFK with her after you cum in her mouth.

There is also extreme snowballing where you suck your semen out of her vagina after BBFS and then proceed to share it with her in DFK.  Obviously rather risky.

If you are considering it, drink lots of pineapple juice for several weeks before the deeds are done to improve the flavor of your semen, as well as staying way from foods like fatty meats and strong spices that tend to cause musky and/or salty flavored semen.  Eating bananas helps too.