There is a right order to warming a cold room, and most people do it backwards: they turn the heat up first and deal with the draughts never. Here is the sequence that actually works, in the order that gives you the fastest result for the least money.
Step one: close the loop before you add heat
A room loses heat through gaps far faster than a small heater can add it. Before buying anything, spend twenty minutes on the obvious leaks: the gap under the door, an unlined curtain over a single-glazed window, an unused chimney, a letterbox. A draught excluder and a lined curtain will do more for a bedroom's overnight temperature than an extra 500 watts.
Step two: shrink the space you are heating
Close the door. Heating a bedroom with the door open means heating the landing and the stairwell too, and hot air will leave upwards faster than you can replace it. If a room has a high ceiling, accept that you are warming the top of it first.
Step three: position the heater for airflow, not for looks
Put the heater where the cold comes from — usually the window wall — so its output meets the descending cold air rather than fighting it across the room. Do not put it behind furniture or under a shelf; the manufacturer's clearance figure exists because blocked airflow trips the overheat cut-out and slows the room down.
Step four: pre-heat briefly, then hold
Run the heater on its highest setting for the first ten to fifteen minutes to bring the room up, then drop to a lower wattage or an ECO mode to hold the temperature. Holding costs far less than climbing, and a good thermostat will cycle the element rather than run it continuously.
Step five: turn the whole-house thermostat down
This is the step that makes the maths work. A local heater added on top of an unchanged central setting is a pure extra cost. Drop the central thermostat a few degrees, let the occupied room take up the slack, and the net bill falls.
What not to do
Do not run a heater on an extension lead or a power strip. Do not leave a floor unit running unattended overnight without a timer and a tip-over cut-off. Do not put a heater in a bathroom or laundry room unless the manual explicitly permits it — most portable ceramic units are rated for dry rooms only, and that restriction is printed on the box for a reason.