How do I gather fabric evenly?

Two rows, not one — this is the part people get wrong.

Sew two parallel rows of the longest straight stitch your machine offers, one just inside the seam line and one just outside it, leaving long tails at both ends. Do not backstitch.

Pull both bobbin threads together from one end, easing the fabric along as it gathers. Two rows keep the gathers upright and even; one row lets them twist and flop.

Distribute before pinning. Pin the gathered piece to the flat piece at the ends and the centre first, then at the quarter points, then even out the fullness between the pins. Gathering evenly means distributing evenly, not pulling harder.

Sew with the gathered side up so you can see and control the folds as they pass under the foot.

Remove the gathering rows afterwards — the one just outside the seam line pulls straight out; the inner one is caught in the seam and stays.

For long runs, a ruffler foot makes a fixed pleat every few stitches, so a metre gathers identically end to end in one pass. Basting-thread gathering over a metre of fabric is where the unevenness creeps in.

On heavy fabric, gather over a cord: zigzag over a length of strong thread, then pull the cord.