How do I approach to create a model like this?
Here is what I do for such shape.
- Use a Bezier curve, in this example called "path to follow" for the general curving shape.
- Use a different Bezier curve, in this example called "bevel shape" for the shape you have drawn in red.
- Use a third Bezier curve, in this example called "taper" to control how the bevel tapers from one end of the path to the other.
After I've created the 3 curves, but before I tune them, I set the Object Data Properties for the "path to follow" curve by going to the object data properties for the curve and adding the "taper" curve to Geometry as the Taper Object and the adding the "bevel shape" Curve to Bevel as the Object after setting the Bevel type to Object:
Here's a funky object I created this way
The "Path to follow"
and finally, the "bevel shape"
Notice that for simplicity that they all sit on the same plane and have the same origin. That's not required. Experiment to see what effect moving and rotating them has.
I strongly recommend giving the curves unique names. It's very easy to screw up and use the wrong curve as the bevel or taper, or to apply bevel or taper to the wrong curves.