Emma went to Hobbycraft and bought some sheets of felt in green and black. I used one sheet to make the hat upright, folding in a 'brim' and making the top slightly wider than the base. Rather than cut anything off I folded the excess down inside, which also reinforced the body. I then took a second sheet of felt and, using pins, fashioned it round to make the overlapping top. You can see it's sort of pleated on. Again, I didn't cut any excess off, but used it to make a sort of padded 'plug' in the top which sits in the top of the body. I carefully stitched it all down (removing the pins, obviously).
The funny thing is that Emma found an old picnic plastic cup and I made the hat to fit round that, so it's reinforced inside. Finally, I partially stitched the hat round the hairband before using a glue gun to secure it on (gluing the plastic cup to the band as it's a stronger bond). The ears were made from a single sheet of black felt, which I cut in half, folded each half over, pinned an ear shape, cut it to size and then sewed round the edge with an overstitch. I then glued each ear to the hairband and then ran a few rings of cotton thread through the top to a) hold the open top of the ear closed and b) to make sure each one was tightly fixed to the band. The band has those little 'teeth' on it most of the way round which helps hold it in the hair, and this provides a good non-slip base for attaching things.
The black waistcoat was a bit of a cop-out. Emma bought a black teeshirt. I cut it short, cut the sleeves out, cut a gap up the front and then cut a sloping front. I then just did a bit of slip-stitching to each side of the new shoulder seam to stop it fraying. As she'll only wear it the once, I wasn't going to spend a lot of time seaming everything. Luckily jersey curls round on itself forming its own tidy hem.