How do I sew a smooth curve?

Sewing the curve is the easy half. Making it lie flat is the half people skip.

Sewing it. Shorten the stitch to about 2 mm — shorter stitches follow a curve more accurately. Go slowly and steer continuously rather than pivoting; pivoting produces a faceted curve rather than a smooth one. Watch the fabric edge against your guide line, not the needle.

Then clip or notch, and this is what decides the result.

On an inward (concave) curve — a neckline, an armhole — the seam allowance is shorter than the seam line, so it pulls. Clip into the allowance at intervals, almost to the stitching but not through it, so it can spread.

On an outward (convex) curve — a collar edge, a pocket corner — the allowance is longer than the seam line, so it bunches. Notch it: cut small wedges out so it can compress.

Getting these the wrong way round is the commonest mistake, and the symptom is a curve that will not lie flat however much you press it.

Then grade the seam allowances to different widths so they do not form a ridge, and press over a curved surface — a tailor’s ham — rather than flat, which sets the curve into a crease.