The simple answer is to cut the angles at the roof / soffit as accurately as possible, and to fix the boards with the bottom edge perfectly level and then install a bead/ripping over the top of the cladding to cover the small imperfection at that junction.
Also, consider castellated horizontal battens behind the cladding to avoid having to batten vertically, then horizontally. You can (probably, depending on local building supply) get fluted PVC battens too, which almost permit 100% of any moisture to vent / drain behind the cladding. That's the best option if you have that available to you.
The simple answer is to cut the angles at the roof / soffit as accurately as possible, and to fix the boards with the bottom edge perfectly level and then install a bead/ripping over the top of the cladding to cover the small imperfection at that junction.
Also, consider castellated horizontal battens behind the cladding to avoid having to batten vertically, then horizontally. You can (probably, depending on local building supply) get fluted PVC battens too, which almost permit 100% of any moisture to vent / drain behind the cladding. That's the best option if you have that available to you.