Place 2 stair blocks side by side for the seat
Place 1 stair block behind them facing the same direction for the backrest
Add slabs on the sides of the seat for armrests
Use carpet on top of the stairs and slabs for a smoother look
Place signs or trapdoors on the front and sides for extra detail
Use different block colors to match the couch style
Add a second row of stairs behind the first if you want a larger couch
Place cushions with wool blocks or carpet on top of the seat area