I cut my fabric 10" x 4" on the bias... By cutting on the bias, I won't have to finish the edges, I can leave them raw & they will fray just a tiny bit. It you use a knit accent fabric, you do not need to cut on the bias.
Use a basting stitch to gather the short edges, you can do this by hand or by machine since it is such a short piece.Gather the edges so that they are the same width as your tank shoulder strap, pin in place. to determine the position, I measured down 6" from the shoulder seam on the front of the tank, then 4" wraps around to the back of the tank. Do the same to the back with the gathers & pinning.