Okay, this is something that drives me nuts. I normally don't post in sexuality, but I feel that I need to now. The point of using your fingers isn't to use them as if they were a penis. I hear from people and read about someone just inserting a finger or two and pounding away at a girl, and you know what that does? Pretty much nothing. That's totally pointless. You use them because they have more dexterity and you could get to exact spots that will stimulate her better. All you need is one finger, very little movement, and the ability to multitask.
Have her lay on her back, have your palm facing up, insert your index finger, and curl your finger upwards. That's the spot you want. It'll be slightly rough. Then rub your finger in a circular motion, bring your hand back and forth only about an inch, or a combination of the two, all the time doing it gently with a little pressure. As you go, increase the speed and pressure slightly. You are now left with one free hand and your mouth/tongue. One should be used on her clit, the other, just be creative. Either lightly touching or licking everywhere still within reach. Thighs, stomach, and breasts being places that tend to work really well. Just experiment, and above all else, HAVE FUN! That's the key.
And if she's that nervous about it, pick your head up every so often and let her know how great she is and how much fun you're having. Just like men, women can psyche themselves completely out of it for various reasons. A lot of times it's either because they're self conscious or they're worried that you aren't enjoying yourself because you're not being stimulated. So just remind her that you are having fun, you're doing this because you want to and because you enjoy it. Keep her at ease.
Another thing, don't go diving straight down. Take your time and tease.
GO SLOW!!! Whether it starts with a kiss that leads to kisses and licks trailed as you make your way down her body or whatever you decide to do, make sure to take your time. Don't go rushing everything. As you get more experieced with being with her, you'll learn each spot that really works well for her and you'll know what will work and what won't.
Wow, for someone who never posts in this part of the forum, I had a lot to say. 