I would suggest making a cardboard template using thin card and trim until you atre happy with it.
The leather will naturally stretch, more so if wet so curves of small radii can be got around easily. When it comes to larger radii which go in multiple directions you may have to slit the leather and pull together.
As an example here is a SIGG bottle I did some time ago with multiple curves.
You can see that the top section was split down into four sections that were pulled together with the threads.