(Oh, and naturally I also have the project posted on my blog.)
Step 1: Select a T shirt
Start with a t shirt with no side seams. Printing is okay, as long as it doesn’t make the fabric stiff or pucker. Also check how the printing looks on the inside of the fabric (the finished tank is turned inside out)– some designs show up through the fabric. Old, beat-up t shirts work great! 100% cotton will work for this pattern, but I think that a cotton/poly blend often has a nicer drape, and is more resistant to unraveling.
Select a shirt that has a width 3-6″ larger than your bust measurement. So: 30″ bust= 36″ t shirt (or 18″ across); 36″ bust = 42″ t shirt (or 21″ across). The bigger the t shirt you use in relation to your size, the more drape the tank will have. This pattern works for size 30-42″ bust. (There is no reason this technique wouldn’t work for smaller or larger sizes, but the pattern pieces will start to be out of proportion. You could also experiment with scaling up or down the pattern pieces for extra small or large sizes.)