First, buy a wire wreath form. Mine had four wire rows and was 18-20" in diameter. The rest is pretty easy. Cut strips of fabric (about 8" long x 2" wide). You can use pinking shears to look extra fancy. You can also use sharp clippers (like you might use for ragging quilts) to make the wreath look "fluffier".
Start tying the strips around the four rows of wire. Use simple knots, but make sure you do them twice (DOUBLE). You need to use A LOT of strips. Here's what it looks like when you are about 50% through. It looks done, but you really need to fill in the strips until it is very tight.
The strips will start to stick straight out as you get the fabric strips closer and closer together. This provides the fullness for the wreath.
Here's the finished product: