How do I shorten a pair of trousers?

Measure with the shoes on, mark both legs separately, and do not trust the pair to be identical.

Pin the length while wearing them, with the shoes you will actually wear and the waistband sitting where it normally sits. Fold the excess to the inside and pin. Trouser length is a matter of a centimetre either way, so this is worth doing properly rather than measuring against another pair.

Check both legs. Legs are frequently a few millimetres different, and so are people. Mark each one where it looks right rather than transferring one measurement to both.

Unpick the existing hem and press the old fold out. You need to see the real edge before you cut anything.

Add the hem allowance below your new line before cutting — typically 3 to 4 cm for a turned hem on trousers, less on lightweight fabric. Cut a straight line parallel to the old hem, not to the floor.

Check the leg width. On tapered trousers, shortening by a lot means the new hem edge is narrower than the fold it has to fit inside. Ease it, or take a small amount out of the inside leg seam.

Finish the raw edge, turn, press firmly, and stitch — a blind hem for tailored trousers, a topstitched hem for casual ones, matching whatever the original was.

Try them on before the second leg if you are unsure. Unpicking one leg is half the annoyance of unpicking two.