Hi, how do I make a wavy rectangle like in the picture (pink) above? I tried using Zig-Zag but it turned everything wavy (yellow) instead of just the sides.
One way for the identical sides case shown is to create one wave side as an open path with the Pen Tool or whatever, then Alt/OptionClickDrag it to the desired position of the opposite side, then join them; you can deselect and SkiftClickDrag over the relevant end Anchor Points or ShiftClick each with the Direct Selection Tool and Ctrl/Cmd+J, then select the whole path and Ctrl/Cmd+J again; there are also other ways.
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As Jacob said, there are other ways.
Aplly the Zig Zag effect to a rectangle (like you showed for the yellow shape).
Direct select and delete 2 opposite sides
Object > Expand Appearance.
While stil selected: Object > Path > Join.
To get an easy regular wavy line, your Zig Zag is the better/obvious choice, but I would delete the ends segments first, to be able to see the effect right away (preview ticked/unticked).
How do you "delete the end segments first"?
For each end segment, deselect (you can click an empty spot) and Click the segment once with the Direct Selection Tool.
You can also just draw a fat straight stroked line and apply Zig Zag to it.
With a rectangle use Envelope distort- Make with Warp. Use the Flag style. Copy and join them together.