Step 1: Use the favorite tank top to create a pattern
First layout your tank top on the newspaper, lining up the edge of the newspaper with the center front or back of the tank top. Trace around the tank (figure 1). Double check that the measurements are at least in the correct ballpark by measuring the unstretched bust, waist, and hem circumferences of the tank top and matching those to the patterns measurements. (This is where I went wrong the first time around, and trust me, not having to take everything apart is awesome, so measure twice and cut once!) Use the curved rulers to smooth out the curved lines of the garment - in this case all seamlines have a slight curve except for the shoulder seams and then cut out the pattern pieces (figure 2). If you plan to add a shelf bra, copy the original front and back patterns to a new piece of newspaper and then cut off the torso to the correct length to go over your bust. Make sure to add an extra 1" to the length for the elastic casing!
Because I knew I wanted to have a contrast material, I drew lines on the front and back pattern where I wanted seamlines, cut them apart and then added seam allowances (generally I add 5/8"). Since we are finishing the neckline and armholes with binding material, do not add seam allowances there. Otherwise, add seam allowances and a hem allowance.