Filing:

Hold the file pointing toward your face (looking down its length), with the finer surface facing upward.

Holding your finger at a ninety degree angle (perpendicular) to the file, place the nail on the surface.

The vector of your finger should be around forty-five degrees to the plane of the file, so that the nail is being filed somewhat from beneath.

File the nail by evenly drawing the file back and forth with the left hand, exerting even pressure and guiding it in place with the right thumb.

Length:

Hold your hand with the palm facing you, fingers extended with the tips pointing upward.

You should see the tips of the nails just peeking past the fingertips (1 to 2 mm past the fingertip is plenty).

Excessive length causes the nail to drag along the string, causing wasted effort and disrupting the timing of alternation and arpeggios.

Uneven lengths are also disruptive; make sure no nail is significantly longer or shorter than the others.

Note that excessive length is common among players with little or no training; the extra length requires less precise nail placement during fingerstroke and can thus make playing seem easier.

If you are significantly shortening your nails, you may initially find your fingerstroke is less accurate.

If so, spend some time concentrating on placing on the exact same spot of the nail for every fingerstroke.

Shape:

Hold your fingers pointing toward you, so you see only the edge of each nail;

each edge will form a more straight or a more curved line.

Now look at the entire nail, from above.

If filed perpendicularly and from beneath, the straight- edged nails should have a more square shape,

and the curve- edged nails should have a more elliptical shape.

If the edge of the nail is straight but the shape is not very distinctly square,

you've probably been filing the nail from its side and the nail will have to grow out a bit to develop more body on the corner.

If the corners of the square shapes are sharply pointed, use the file to round them (but don't alter the basic shape).

Surface:

After the nail is filed, its edge must be finely polished.

Take a small portion of 500 grade open coat sandpaper and rub, with a back and forth motion,

the same spot of sandpaper on the edge of the nail, particularly concentrating on the left side of the nail.

The surface of the sandpaper will wear down as you rub, creating an increasingly smoother polishing surface.

Keep rubbing until the edge is as smooth as a glass surface.

Above information extracted from: http://www.guitarsite.com/nails.htm

I know right, it sounds kinda confusing, the girls should generally get it though.