When gathering and aligning the seam edge to the plain edge underneath, distribute the gathers evenly with your fingers as you pin the seam edges together. While using your fingers will give you a generally even distribution, using a pin to even out the gathering folds or ndividually, prior to machine stitching the seam will result in a perfectly balanced, even gathered seam.
As you stitch the gathers, you’ll find that the pressure of the presser foot will somewhat distort the gathering folds, disturbing the evenness of the gathers. It may help to continue evening out the gathers using a pin in front of the presser foot as you machine stitch.
You may find that just using your fingers to keep gathers flat and evenly distributed as you machine stitch is a more controlled, tactile way to keep the gathered folds in place under the presser foot feed.