Draw the outline of the Pokeball onto pattern paper, allowing space for a seam. I used a plate as a stencil
Add the details. Again I used objects to help get the shapes right.
Use the pattern to create the patterns for each individual piece and cut them all out. Ignore the furthest left piece, that was my mistake.
To figure out the length of the middle strip, measure the radius and multiply it by 2pi. Remember to leave seam allowance. The width of the strip will be the width you want the bag plus seam allowance.
Start cutting the pieces and sewing them on. It doesn't matter what colour thread you use for the large red and white pieces because you will not see it.
Your pokeball should now look like this (sorry I never took a pic after I sewed both pieces). Do the same for the back if you want your bag's design double-sided.
Sew the black part on with black thread and then the white part on top with white.
Next, take the strip that will be holding the two sides together and insert a zip.
Sew the zip onto the two sides, add the straps (I unfortunately didn't have anything I could use and didn't have time to run to the store so I used a long shoelace!) and you're done!