Start by cutting fabric in strips - between 3" and 5" is effective and gives you room for seams even with quite a wide curve.
Select two strips of contrasting fabrics, iron them and place them one on top of the other, both right side up. If they are not the same width then put the widest strip at the bottom.
You now have two pairs of fabric strips: the top strip from the right with the bottom strip from the left and vice versa. Pair them with their right sides together.
You will need to ease the edges to match. Some people prefer to pin first, others match as they sew. I find it better to pin as I tend to panic if I'm trying to match curving seams as I machine! You don't need to clip the curves at this stage but you might need to clip a little once the seam is machined if it will not lay flat. Jenni's advice was don't clip unless necessary.
Once the seam is machined, open it out and press carefully, clipping as required, you have a wavy seam! (And crikey, what a state my cutting board is in!!)
This is how it will sit in the quilt I'm making today:
Once wavy seams are mastered there is no end to their use - what about a simple nine-patch with wavy seams, or what about wavy log cabin?