FIRST you need to start with a shirt that is a little too big. A tight fitted shirt will not work, the scrunching will bring it down about 1/2 a size, depending on how much you do.
now after you take it off, outline with a fabric marker or chalk the area you want to work in.
If you have never bought elastic thread this is what it looks like, you will find it in the elastic section. Hand wind your bobbin not tight just pulling a little. Then you will use it just like you would a regular bobbin. Use matching thread on top. Then start sewing.
Here is an example of a pattern I chose. THERE IS NO EXACT PATTERN OR PROCESS TO THIS. Just sew in zig zags or figure 8's until you fill the area. This is the way I ended up doing it because there was less turning, lifting foot, and backing up. It was super easy. Believe me, cause I don't do hard stuff. --The only thing to watch for, is trying to keep the circle areas around the same size, that is where the fabric will pop out.
This tank was SO baggy I decided to do a middle section, I really like it on my bathroom floor, not sure how I like it on me. I added little ribbon flowers to disguise how some of the areas are a little off centered.