fatzilla

http://m-sewing.com/patterns-catalog/women/vests/vest-5017.html that one is okay, but I like how this one is more detailed? Also, it has the shape that you want. While it looks like Luffy's vest it has a more feminine shape. It says use silk or satin but for Luffy I'd just use a broadcloth or any kind of woven fabric (Denim, button-up shirts, not stretchy) because silk and satin is slippy and shiny, jersey (t-shirt material, sweatpants material) stretches too much.

It's not necessary for a simple vest, thankfully. For a vest usually you will have the outer fabric, an inner facing, and a liner. Do you have a sewing machine? You'll need a straight stitch so it doesn't have to be a fancy machine, just basic. You'll also need a good vest pattern. Haven't looked but you might find one online possibly for free. :)

Now I feel bad. I didn't mean to make you nervous. Oops.

Oh no, I'm so sorry. I was just teasing you.