Six steps, in this order

Start with the lamp

Choose the light before the furniture. One warm table lamp at seated height. If you buy nothing else, buy this.

Fit a dimmer

Every light in the room should go lower than you think you want it.

Darken one wall

A single dark wall does more work than four. Deep green, ink or oxblood, matte.

Hang something heavy

A weighted curtain, even on an interior wall with no window. It kills the echo, and the echo is what makes a room feel fast.

Remove the overhead bulb

Not switch it off. Take it out. Nothing undoes the effect faster than a guest reaching for the wall switch.

Add one old thing

Aged leather, unlacquered brass, a worn rug, a chipped decanter. It has to read as inherited, not bought.