For gathering: a sewing machine with a long stitch length (5 or 6)
A pattern that calls for ruffles or gathered pieces.
Set your machine top tension at about an 8 (1-10). This part is adjustable to the thickness you want your gathers. The higher the number the tighter the gathers. Adjust on the fly.
Put right sides together with the gather edge matching the edge you want to sew it to, and Stitch.
Repeat if necessary for multiple tiers.
If you are making a tiered skirt, start with the bottom tier and work your way up.
For a skirt, stitch the side seam.
For a dress, stitch from the bottom of the skirt up through the sleeve.
For a top, stitch from the bottom of the top up through the sleeve.
For pants, stitch from the ruffle at the bottom of one leg up past the crotch seam and down through the ruffle at the bottom of the other leg.
Sewing ruffles on a flat garment, makes them a breeze. Easy Peasy.....let's see some ruffling!