How to gather knit fabric
Gathering knits with floss is a great method for beginners who haven't sewn with elastic or people who achieve perfectly spaced gathers. But that method isn't entirely reliable and without a lot of redistributing and patience my gathers wouldn’t always stay evenly spaced.
The best option would be to use elastic and you'll get a perfectly gathered waist.
Find the how to gather knit fabric using elastic method here.
Tutorial by Allie @ Indiesew