How do I sew stretch denim?

Stretch denim is a woven with elastane in it, so it needs woven techniques plus a few knit precautions.

Use a stretch needle, or a jeans needle in 90/14 or 100/16 depending on weight. A stretch needle is the safer choice on anything above about three per cent elastane — a sharp point can cut the elastane and leave a line of tiny holes that opens up later.

Use polyester thread, which has some give. Cotton thread has none and snaps along a stretched seam.

Stitch length 2.5 to 3 mm. A very short stitch has no give and will break before the fabric does.

Stabilise anything that must not stretch. Waistband, pocket openings, fly and yoke seams all need a strip of fusible interfacing or stay tape, or they grow in wear until the garment no longer fits.

Prewash it. Stretch denim shrinks and it also relaxes — a garment that fits perfectly off the machine may be a size looser after an hour of wear. Sew it slightly closer than feels right.

Topstitch with topstitch thread and a topstitch needle, and lengthen to 3.5 mm. That is what gives the ready-to-wear look; ordinary thread reads as a repair.

At thick seam junctions, hammer the folds flat with a mallet, level the presser foot with a folded scrap, and turn the handwheel by hand over the worst points.