The advantages of full skirts are obvious. They spin and twirl in a delightful way, and have been in style for the last few summers, usually in the form of a ruffled peasant skirt. Even when they aren't fashionable, they can be useful for all kinds of dance (salsa, flamenco, swing, modern, belly), as well as for costuming.
If you have basic sewing skills, they're incredibly easy to make. There are two popular ways to create a very full skirt: make a circle skirt, or make a gored skirt with wedge-shaped panels.
To make a circle skirt, in the most simple terms: draw, on fabric, a circle the circumference of the intended wearer's waist, then another circle as far outside of it as you want the skirt's length to be. (If that doesn't make much sense, think of a "bull's-eye" pattern.) The result is a typical circle skirt. The interior circle is the waist, which will need a waistband, and the exterior circle is the hem, which will need to be finished.
Getting the best possible results is a little more complicated than that. For more circle skirt tips and tricks, as well as everything you need to know about gored skirts (and a few useful pattern links), please join me after the break.
Sometimes it's helpful to break a circle skirt up into wedges: it depends on the measurements involved. If you want to make a long skirt of this kind for an average-size adult, it's more "necessary" than "helpful," because such a skirt requires more material than a single width of standard fabric will allow.
The invaluable diagrams and instructions for a circle skirt at Madame X's Costuming for Middle-Eastern Dance will show you how to layer and fold the fabric so you get a complete skirt. The page also includes other suggestions: a double circle skirt, for example, has even more swing and flare.
But a skirt with a hem that is twice the circumference of what a circle skirt would be at that length is often a hybrid of the circle skirt and another kind of full skirt, the gored or paneled skirt, which gets its fullness from tall triangular wedges inserted into the hem.
To make a gored skirt, use two rectangular panels of fabric to create the front and back of the skirt. Each should be about half as wide as the intended wearer's waist measurement. Triangular panels, or gores, will be added at the sides to create enough ease for the wearer's hips and a swingy hem.
These skirts are explained in more detail at Elaine's Dance Costume Page, where there are diagrams and a pattern calculator for gored skirts. Elaine's page also has instructions and a calculator for a circle skirt.
More and more fullness can be added by inserting more and more gores, though they'll look funny if you don't insert them evenly and symmetrically (the exception is the back, which may be able to survive having a single centered gore, depending on what the rest of the skirt looks like). This should work with either kind of skirt.
No matter what kind of skirt you want to make, both the sites I've linked here suggest that you try the patterns out for the first time on fabric that you don't care about very much. That way, you'll know if you're going to have problems before you cut into a length of silk that sells for $50 per yard. Good plan!