1) Create boundaries. Try to separate your outdoor space from the rest of your yard. Carve out a space using potted plants to create the illusion of walls. Hang a sail cloth 'roof' or even outdoor drapes; a trellis will give the appearance of a doorway! Trees and plants can serve as your boundary and enhanced with LED string lights. Define the seating area with hardscape like gravel, pavers, or a rock border, or simply lay down an area rug. Many stores sell easy-to-assemble decking that locks together creating a patio. Using polypropylene rugs is ideal for outdoors, but a covered area can tolerate sisal, cotton or other blends. Umbrellas are handy 'roofs' and also serve as a place to hang lights.
2) Add light. Candles are the simplest way to bring light to outdoor rooms. Iron stakes with candles under glass holders, torch-type lanterns, and hanging candle lanterns are all easy to find and use. String lights can be hung from trees, the trellis, or even from yard stakes and install quickly to provide instant, warm light. If you have a structure, consider installing an outdoor rated fixture or even a hanging iron candelabra. For tabletops and to light the paths, use hurricanes with real or flame-less candles, candles in jelly jars, and floating candles in dishes. Consider safety first in what you choose.
4) Soften it. Using fabric - pillows and seat cushions - will make the space cozy, even if you opt for plastic stackable chairs. Sun resistant fabrics are easy to find in ready-made cushions and pillows. Handy at sewing? Your fabric options are almost endless and you can customize the style and patterns. Use tablecloths and colorful napkins to dress your table; cotton is easy to wash. The sight of a covered table is inviting and you can customize with fabric remnants, layered or used one at a time. Skirt a bench for a dressy look. Hang drapes where you can. Polyester sheers are inexpensive, and even if they are used in a non-covered area, may be able to be washed and re-used. On partially covered patios and balconies, I use cotton or cotton-linen panels that are easily taken down at the end of the season and used for years.
5) Bring on the decor. Add decorative elements as though designing an indoor room. Hang elements on a wall, from a tree, or even from a stake. Glass hurricanes look wonderful filled with shells, plants, or just candles. Ceramic bowls, vases and planters will make the space look finished and may also be used for food, flowers or just decoratively. Tables look great with centerpieces so be creative in what you use. I have a plant stand that I layer with plates, and fill the plates with seashells, candles, or plants. Baskets work well to store items like extra pillows, a throw, or magazines and books. I have a lidded basket that's large enough for seating, and topped with a pillow, becomes an extra seat.