With nothing selected, apply the Bounding Box preset from the Effects & Presets panel.
Parent the bounding box to the layer you would like to create a bounding box around.
Under the Bounding Box effect you can customize the appearance of the bounding box.
When you initially add a bounding box there will be an expression error until you parent it to another layer.
If you are still getting expression errors, make sure your expression engine is set to JavaScript.
File > Project Settings > Expression Engine > Expressions > JavaScript
You can create a bounding box around a pre-comp. Or you can use Get Rect Pro to create a rectangle around multiple layers, then create a bounding box around that rectangle.
