>>2928299
start flat, apply precut accents, plasters and sand
i think you could paint them just like anything else i would repair with drywall mud and a cardboard jig. take some cardboard get a soda can washer something round trace it cut it out and use to smooth drywall mud then sand and paint.
paint the walls first when it is flat apply cornice or molding or dowels or w/e then paint trim. pic related
I think /diy/k is covered in bot post, peace out cupcakes.