This is it! It is just a tube of fabric with arm holes cut out of it. You will want the grain of the fabric to run length-wise so the selvage is at the neck and the waist. Make sure that you don't cut the arm holes too big! They will almost double in size once they are are hemmed, so start off with small holes and experiment when you flip and pin them for the hem.

Creating a more snug cowl neck is an option that I tried with this particular tank, although you don't have to do this. The picture above in the first step is really all you need to do.