Use a hand needle, matching thread, scissors, and the garment. Keep needles away from children and account for the needle when finished. Practise on scrap fabric first. This guide covers an ordinary sew-through button and a short open seam in stable woven fabric.
Turn the garment inside out. Align the original seam edges, keeping the garment’s outer faces together. Start slightly before the opening. Use small backstitches along the original seam line and continue slightly beyond the gap. Secure the thread and turn right side out.
Stop if fabric is tearing around the repair, a zip is involved, or the material needs a specialist method. Reinforcement or professional repair may be more suitable than pulling stitches tighter.
On scrap fabric, mark four points A–B–C–D along a line, about 3 mm apart. Bring the needle up at B, down at A, then up at C. Go back down at B and up at D. Continue: each new stitch goes back to the previous point before coming up farther along. The short stitches join on the visible side. Keep fingers clear of the needle path.