This is my favourite method for gathering fabric. It took me a few years of using other methods and loads of unpicking to realise this was it.
It may look fussy BUT IT WORKS!!
1. Sew three rows of machine basting on gathering section, leaving long ends.
1st row; 2-3 mm from seam edge. This row of machine basting holds seam allowance together to make for easy handling when overlocking or binding.
2nd row; 2-3 mm from seam allowance. This row holds gathers together near seam line.
3rd row; 3-5 mm past seam allowance. This row holds gather down to stop them sliding sideways into your seam line
2. Tie together the top threads at one end of the stitched rows.
3. Pull top threads all together from other end.
4. Gather to required length.
5. Tie off loose threads.
6. Distribute gathers evenly by hand.
7. Machine baste another row - near seam line to hold distribution of gathers in place.
8. Straight sew to joining piece.
9. Overlock or bind any raw edges.
10. Unpick the visible 3rd row of machine basting.
11. You are done!