Ok, so first put your tank top on and mark where it would fall under your bust line. If you are serging the cut edge add about a 1/4 inch to your length. If you are using a regular sewing machine add about 1/2 inch so you can fold it under about 1/4 inch twice and hem.
Then, place the elastic band under your bust and add about an inch to the measurement. I know my elastic is white here but I had a change of color choice at the last minute!!
Then, take your fabric and either hem (folding the fabric over 1/4 inch two times) or serge the top edge…where the elastic band will meet it.
Then, pull the bottom thread (or the bobbin thread) and make nice gathers throughout the fabric until the length measures your elastic band.
Then, just pin the skirt fabric, right side facing the elastic band, to the bottom of the elastic band and zig sag stitch it in place. Make sure your skirt seam and your elastic band seam are matched up too!
Then, pin your tank top, wrong side out, at the upper portion of the elastic band and zig zag into place too. Making sure your tank top side seam is matching the elastic band/skirt seam too!!