Glue the photo or paper on to the magnet. I set them under a heavy book to dry flat for a few hours. Then Mod Podge over the photo or paper & let dry. You may need to trim it up a bit, if the edges didn't line up perfectly.
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.
I used an elastic piece a few inches shorter then the sleeve/shoulder measurement. I pull the elastic when sewing it to the inside of the shirt to create a bit of rusching.
I cut off the neck trim from the shirt I sewed the flowers too... made it kind of a boat shaped neckline. You can just leave it a raw edge, knit will not fray.I added a few "stems" under the flowers, using just strips of left over fabric. I sewed down 3 & left 3 hang loose.