This is another method that I have picked up on the web. I think I found something similar when I was trying to make a tu tu. Then I was trying to insert 4" of narrow elastic in the wrist of a puffed sleeve and I just couldn't stretch it far enough. Zig zagging over the elastic without touching it allows you to pull up the sleeve very tightly without having to stretch the elastic at all.
Without cutting the elastic measure off what you need - 13cm (5") for a panties leg, 11.5cm (4 1/2") for an ankle, 9cm (3 1/2") for a wrist, 10cm (4") for an upper arm.
Mark the length with a pencil dot.
Mark a guide line for the elastic on the garment. Pin the end of the elastic and the raw edge level.
Secure the elastic end with a few 2mm zig zag stitches. Back stitch for extra hold. Widen the zig zag to about 6mm so it just clears the edges of the elastic. Stitch to about an inch away from your pencil mark.
Leave the needle down and lift the presser foot. Gently pull on the elastic in front with your left hand and behind, hold the fabric between the fingers of your right hand so the fabric bunches up and the elastic slides forward.
Stitch again. At the end line up the pencil mark with the edge of the fabric. Zig zag to the end and secure the end of the elastic with a few narrow zig zag stitches. Back stitch for good measure.
Even out the fabric gathers.