Photograph the room in daytime from the corner opposite your main seating, so the sofa, the window, and the floor are all in view. Turn off harsh overhead lights to keep colors accurate. If the room has a fireplace or a bookcase, keep it in the frame, since features like these anchor a cozy layout.
Pick a warm style like Japandi, Scandinavian, or Boho and write a short instruction about mood and materials: "keep the windows, fireplace, and wood floor, add a wool rug, a deep cream sofa with knit throws, two table lamps with linen shades, and paint the walls a warm clay color." Mention the lighting you want, because warm lamplight changes the result a lot.
Create two or three variations: one with a warm wall color, one with white walls and heavier textiles, and one with a reading chair in the corner. Compare them and note which changes add the most warmth for the least effort. Often a single wall color or rug turns out to be the change worth making first.