I love how the plot part of that game uses a dating-sim engine. I kept hoping that if I performed surgery really really well, the blonde chick would be impressed and disrobe. But I digress...
You might need to salvage the buckle from a cheap bookbag or something. Some fabric stores sell them, but not all.
Most of it needs to be sewn together. The fabric is again, gonna be the synthetic nylon (I think it's nylon...) used to make bookbags. All edges must be folded over and sewn down because the stuff frays like madness. Cutting the fabric with pinking shears isn't a bad idea either. Since it's just a costume piece, the underside can look horrible; otherwise, you'd do extra techniques to keep it all pretty.
The larger piece that is under the main strap should be a padding, made by wrapping fabric around a piece of sheet foam. Craft foam is fine.
The upper strap should probably have some woven bookbag strapping hiding underneath for strength. The fabric overtop it should just be decorative and not really bear much stress.
To make the pouch you stiffen it by gluing a piece of card-stock to the fabric leaving seam allowance on the edges, and the folding it around in a loop so it forms the front side, the bottom, the back side, the top, and the flap. Then you sew up the sides. Actually, this can be done with a single piece of fabric if you properly work out the pattern. You probably want to stitch the belt-loop onto the pouch before adhering the cardboard. To hide the seams, you need to sew it together inside (cardboard side) out and then turn the outside in afterwards.
You can find little sewing tutorials on how to make Pouches. I seen some sold at fabric stores, and others in beginner sewing books. They should give you a much better explanation and better tips for working out the pattern and steps involved. I'm afraid most of my pouchmaking experience is in working with heavy tooling leather, which is a fairly different animal.
The red cross can be painted with simple red acrylic paint. For best results, lay the whole thing out with masking tape. Remove the tape while the paint is still ever so slightly wet.