So many ways you can make a pincushion, its all up to your personal choice. I made one for myself & here is how I done it.
With a scrap of fabric about 3.5"W x 9.5"L, I folded it long ways,right sides together, then sewed up both sides & ends, leaving about a 3" gap in the center for turning & stuffing. Turn right side out & press. Fold it in half to mark where you want it to sit on your wrist. Then about two inches from each side of your mark, zig zag stitch to keep batting & cardboard in place. You can even top stitch at this point, which I did not.
I cut a piece of cardboard small enough to fit , grabbed my batting to stuff it & as you can see in the pic I placed the cardboard in the gap & filled it with the stuffing. Then close up the gap. You will need Velcro, snaps, whatever you want to use as a closure. You don't want it falling off your wrist, lol. I used Velcro on mine, it was fastest & easiest.
You can make this however big you like with any design too. Its all up to you!