If you have basic sewing knowledge and can operate a sewing machine, you can make a new tank top using an old tank top as a pattern. The first step is to find a favorite tank and use it as a pattern to make more. Find fabric similar in style to the tank top you're copying.
Take your old tank top apart. Cut it apart at the seams to create a front piece and back piece. Depending on the style, you may also have ribbing around the top and/or the bottom. Cut very carefully -- remember these pieces are your guide or pattern.
Take your tank top pattern pieces with you to the fabric store. Choose fabric that has the same amount of stretch as the "pattern." To check for stretch, lay the old fabric on top of the new fabric. With both hands, pull slightly to be sure it is the same stretch. The stretch goes around the body. If you need help on the amount of fabric needed, check with a store attendant for advice. Most stretch fabric is sold in 65-inch width. Choose fabric for ribbing if needed. Purchase matching thread.
Wash your fabric and dry it in the dryer to prevent shrinkage later on. Lay it smoothly on table and place the pattern on top. Make sure the stretch of the fabric goes in the right direction. Pin the pattern to fabric.
Add 1/4-inch while cutting to allow for 1/4 inch seams. Add 1/2 inch to length. Use a fabric marker to draw the cutting lines. If your tank top will have ribbing, lay it out so the stretch is the correct direction.
Use a needle made for stretch fabric. Turn your dial to appropriate stretch stitch. If your tank top is to have ribbing at top or bottom, sew this to the top first, with right sides together; fold ribbing over to wrong side. Stitch-in-the-ditch, which means sewing in the seam so stitching is barely visible.
With right sides together, sew tank top front to back with 1/4- inch seam.
Turn up hem and stitch; don't pull on fabric too tightly.