still problems with "blend"

 From:  mike (MIOHN)
Hi Michael,

its from here:

So he built that profile with separate curves, jointed the parts
and rebuilded it.
But as I see now, when I look close, that ist overall smooth.

But what should I do, if I have a model with an open end
and I want to f.e. close that opening when it is not planar?
How should I rebuild the edges of that opening?