At once or twice a year, buy off the rack and pay a tailor. And the thing that makes that advice actionable is knowing what a tailor can and cannot change, because that decides what you are shopping for.
Cheap and easy: trouser length and taper, sleeve length, taking in the waist of the jacket, taking in the seat and thighs. These are routine and transform how a suit looks.
Possible but real money: jacket length, narrowing the shoulders, reshaping the chest. Some of these are close to rebuilding the jacket.
Effectively impossible: making the shoulders wider, and changing the chest by much.
So the rule when you try things on: the shoulders and the chest decide the suit, everything else is adjustable. The shoulder seam should sit where your shoulder actually ends, and the jacket should close without pulling. Ignore the sleeves being too long and the trousers pooling on the floor — that is the tailor's job and it is cheap.
Budget for the alterations as part of the price from the start, not as a surprise afterwards. A mid-price suit that fits your shoulders plus alterations beats an expensive one that does not, every time and by a wide margin. That is the entire secret and it is why the advice is so consistent.
Two months is comfortable time. Two weeks would not be.