I have two minutes…
Sew a row of stitches inside your intended final stitch line, in the seam allowance. So if you’re sewing the seam at 5/8th, sew the gathering stitch at 3/8th.
Next, pull one of the long ends of the thread (classically this is the bobbin end, but I don’t think it matters. DON’T PULL BOTH – pick one).
You then continue gathering as much as you like or smosh the gathers a bit flat, whatever you need to do.
Easy peasy lemon squeezy. Practice a bit, you’ll get the feel for it in no time.
Helpful hint: do not try to run a gathering stitch longer than about 2 feet long to start with, they tend to give up the gathering ghost around 18″ or so. Also you will “scoot” the gathers along a long gather with your hands, it won’t do it for you.