You may start with the smallest or largest wave just drawn as a simple normal path with the Pen Tool to get the basic shape (of the spine), using just two Anchor Points, one at either end. The key is the direction and length of each Handle.
Depending on version you may use the Width Tool or an Art Brush to give the varying width.
After that you may move a copy and change its size, then Object>Blend>Make, using Specified Steps in Object>Blend>Options.
You'll have to learn it.
See this video, it has an introduction: http://terrywhite.com/how-to-use-the-pen-tool-in-adobe-illustrator-pho toshop-and-indesign-cs6/
Here is a screenshot of a simple wave with two end Anchor Points, not quite the shape you are after; you can see the Handles.
If you have a desired shape, you can lock it and draw on top of it. Just ClickDrag with the Pen Tool at the start so you get the right direction of the Handle following the original. Then ClickDrag on top of the end point in the same way. You can Direct Select the Anchor Points and drag at the Handles by turns until you are satisfied.
To get a general feeling, you may just start ClickDragging more or less randomly and study how the combination of position of Anchor Points and Direction/Lengths of Handles influence the shapes. You may say that you literally drag the shape by the Handles.
With the Pen tool still active, hold down Cmd/Ctrl and drag the handles around. (This is equivalent to switching over to the Direct Selection tool.)
See what happens to the curve.
Conversely you can also drag on the line itself.
With the Pen tool still active, hold down Cmd/Ctrl, drag the path segment and see what happens to the handles.
(Hi Jacob and Peter)
Can i just use arc tool?
The Arc tool is one of the most useless tools in the box.
Forget about it and learn to use the Pen properly. Once you’ve got the hold of it, you will find it both versatile and useful.
Experiment with click, click-drag, double-click-drag and drag with the Option/Alt key held down
Study particularly the behaviour of handles. Long handles and few anchors make for smooth curves.
Take a look at some type that has been converted to outlines and see how the anchors are positioned for maximum effectiveness.
And (unlike this case where you know where to put the Anchor Points) for cases where you may adjust the position of the current Anchor Point, the Spacebar is your friend: you may ClickDrag to place and drag initially, then press the Spacebar while still mousing down on the Handle (or is it mousedowning) to drag the Anchor Point freely about while freezing the Handles, and in that way you may fine tune both Anchor Point and Handles underway, instead having to rely on subsequent Direct Selection which may (adversely) effect the following segments.
Sometimes concision is imprecision. I meant and should have said The Arc Tool is rather restrained and only curves one way at a time.
Edit: Hi Peter and Steve.