It all starts with research:
For a start, you'll need empty weights (in kg), thrust (in nm), fuel capacity (in kg). Then you'll need wing & horizontal stab span dimensions, along with , root, tip, & centerline chords. You'll also require leading edge sweep angles, and lifting surface areas.
Then after performing the necessary calculations, you'll have a baseline that still requires tweaking (lift and drag tables, etc).
Next, plug in the stall values , which are best obtained from flight manuals (if possible), though most Russkie birds have the values denoted on their AoA gages....very thougtful of them
I strongly suggest that you download Kreelin's aeroconvert. Its in our downloads section.
PM me if you need any help.