If you want to wet shape the leather, it's best to use brand new veg tan leather. Most of the used leather items you're likely to come across are made from other sorts of leather, and even if you do find something made of the right material, these items tend to absorb oil unevenly from being handled, which complicates shaping and dyeing.
It might be possible to stretch the right piece of leather into that shape (excluding the bat ears,) but you might be better served doing it in at least four pieces - two for the crown, stitched together up the center like it's done in the drawing, and two side panels for the ears and cheek pieces (or alternatively, one long strip that goes around the base of your skull). The goggles and the ears should hide all of the seams that don't show in the drawing.
You'll want to ask for 3-4 oz. leather - that ought to be thick enough to hold a shape well, but not too heavy. If you do it in multiple pieces, look for leather that's from the cow's back - it stretches less than belly leather, but is more likely to be of a uniform thickness, and will shrink less and stiffen up well.
Test your pattern out in craft foam of about the same thickness as your leather before you cut. Be a little generous in your measurements, because the leather will shrink just a little when you wet it.
To wet the leather, fill a basin with water that's too hot for bathing, but not so hot it would burn your skin if you briefly dipped your hand in. If your water is too hot, the leather will get hard and brittle, and you won't be able to wet it again to reshape it if you dislike the result. Put your leather in the water, leave it in until it stops bubbling, pull it out and pat it dry with paper towel. Now it should be ready to shape.
Ideally, a form made of some waterproof material would be best here, but you can shape the leather by hand. The leather should stay workable for a couple of hours. Once you've worked it into the shape you want, leave it to dry for about 24 hours. If you want to reshape it, you should be able to repeat the process a couple of times without damaging the work, provided your water wasn't too hot.
It would also be useful to score a shallow line where you want your stitch to run at this point, while the leather is damp. That will help guide the awl, and will protect your stitches from wear by keeping them from standing proud of the leather. Once the leather is dry, you can punch pilot holes wth an awl, and stitch the pieces together.
If wet shaping sounds daunting or your budget won't allow veg tan leather, another way to go about it might be to buy a black leather garment from the thrift store, do the side panels in plastic card or something else thin and rigid and then upholster them in the black leather. Then you could do the top panels without any rigid backing, and sew the whole affair together like an aviator's cap. If you didn't plan on doing much leatherworking after this project, that method has the benefit of requiring fewer tools, and might match the motorcycle jacket more closely.
