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Step one: define the subject and distance
Start by deciding what must remain sharp. In a portrait of someone standing by a brick wall, the face and shoulders are the subject; the wall is background. Look at depth: the wall directly behind the person should be only mildly soft, while distant objects should be softer. Zoom to full size and note hair, glasses, hands and clothing edges. Creating a rough plan before touching sliders prevents the common mistake of applying one flat blur to everything behind the subject, which can look pasted on and obviously artificial.
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Step two: build a clean mask
Create a selection around the subject, then refine it at high zoom. Use automatic subject detection if available, but inspect the result. In the portrait, check wisps of hair, the gap between arm and torso, and any glasses frames. Feather the mask slightly so the transition is gentle, then add a separate rough area for foreground objects that should also remain crisp. Invert the selection only after fixing errors. A tidy mask matters more than a strong blur, because halos around the edges are what viewers notice first.
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Step three: graduate the blur and check it
Apply blur to the background in layers rather than once. Add a light blur across the whole background, then increase strength with distance using a gradient or depth map. Keep bright lights rounded and slightly glowing, as lens bokeh would be. Add a touch of matching grain so the softened area does not look cleaner than the subject. Finally, view the image at normal size and at a reduced size; the subject should stand out immediately while the wall still feels like a real place behind the person.