One low lamp beats any amount of decoration
If you change nothing else, change the light. The overhead bulb is the enemy of a cosy corner; it flattens everything and signals to your body that the day is still in progress. A single lamp placed below eye level — on a side table, a stack of books, the floor — with a warm bulb will transform a corner more than a trolley full of cushions. Two small light sources at different heights are better still, because the shadows between them are what makes a space feel soft.
Candles are the obvious addition, and they do work, but a reliable lamp you can switch on with one hand at the end of a long day gets used far more often than a candle you have to find matches for.
Layer three textures and give it a surface
Cosiness is tactile before it is visual. Aim for three different textures within arm's reach: something woven (a throw with weight to it), something soft and deep (a cushion you can actually lean into, not a decorative square), and something natural and hard (wood, ceramic, a basket). A rug underfoot, even a small one, insulates the corner acoustically as well as physically — fabric absorbs the echo that makes a room feel unsettled.
Then give the corner a surface. This is the detail that separates a retreat from a chair. Somewhere to put a mug down without standing up is the difference between sitting for five minutes and sitting for an hour. A small stool, a tray balanced on an ottoman, a stack of hardbacks — it only needs to hold a cup, a book and a pair of glasses.
