The Tongue
I want to repeat, there's almost nothing you can do with your tongue that won't feel terrific, so relax! I promise, you may be confused and uncertain, but she's in heaven. Any licking and sucking of her lips, cunt, or clit are going to feel just great, and I'd no sooner tell guys to "do it exactly like this" than I would tell every chef to follow the same recipe. But for those who are compelled to RTMF, here are a few techniques that you might like to try:
Play around in the vestibule, try lapping her pussy from her cunt up to her clit, leaving your tongue soft and jaw relaxed. This is a good way to start your tonguing. Run your tongue between the inner and outer lips on one side, while holding the two together with your lips. Good job, now do the other side. Fuck her cunt with your tongue - in and out, around and around, etc. This feels nice. Not wonderful or incredible or earthshaking, nice. Keep your tongue in her cunt for a while, moving the tip around and around and . . . That feels better, now lick up to her clit again and kiss it, watch her shudder. Feel the texture of the tip of her clit with your tongue, notice the taste of it. Spend some time slowly licking your tongue over all of her pussy, savoring the various tastes and textures.