How should I decorated a large square living room?

Send some pictures

Paint two-tone walls. While tall potted plants are great for drawing the eye up and accentuating high ceilings, sometimes we crave the opposite effect. Painting color on your walls only part of the way up creates a cozier feeling, tricking the eye into thinking the ceilings are lower than they are.

Use wainscoting as a natural guide or simply tape off a line and paint everything below it.

All depends on whether you are looking to make the space look big in which case I would use a light neutral colour on each wall. You could have one wall or two opposite walls in a feature colour or the same but with wallpaper. I don't think a loud patterned wallpaper looks good on all walls as it does tend to make the room look smaller

You can either wallpaper or emulsion the walls.

Always use a good quality paint and good brushes to avoid brush marks and stray bristles. Try using a good quality roller as well. Keep the room well ventilated too.

Fashionable these days are 3 walls emulsioned and one wall papered as a feature wall

I can provide many decorating tips - too many to post here! If you need advise for a particular aspect then feel free to contact me. Otherwise I would recommend a look through the many YouTube videos on the subject as I simply could not compact it all in to one short message! Two useful tips I can fit in here are 1. take your time and 2. if you cannot afford a professional for all the work then why not have them do the stuff you are not comfortable with, say you do all the painting and have a professional paper the feature wall. Or for difficult to reach stair walls have the professional do the emulsion and you do the woodwork.

start from the window to hide joints.