You need to be frank with her by owning up that you actually do not know how to tongue kiss. You should actually tell her to teach you how to do it since she is your partner. You actually need to go with flow and not rush or be a shovel tongue.
Steps on how to tongue kiss 1. Begin lightly kissing 2. Pull girl a little closer to yourself 3. Kiss a little more forceful but not to much 4. Pull girl even closer (now almost in a hug) 5. Begin to move tongue toward edge of your mouth but not into the girls mouth 6. Pull girl really close (really tight hug(should feel her body heat she's so close)) 7. Move tongue into girls mouth and gently wiggle it around 8. Move tongue to top of girls mouth and rub gently 9. (These next few are optional) Move hands toward anywhere on the girl that both you and her are comfortable with like the waist and back or somewhere you know she won't mind 10. Slowly move hands to more romantic position such as butt, chest or neck 11. Pull girl even closer with your hands rhythmically rubbing against her butt and chest or neck 12. Good Luck