Grab either the top three threads or the bottoms three threads and pull with one hand while pulling with the other.
Iron the piece by going up to the gathers, but do NOT iron over the gathers. This will squish them and ruin all your hard work.
Ratio 3:1 on grainline gathers.
I hope all this makes sense! Have fun with your gathers! There are so many things you can do with gathers. Like gathering lace. . .