Unscrew the top of the umbrella. Pop off the short wires that hold the ribs to the part that slides up and down the stem of the umbrella - they are like staples, and you can use your pliers as a staple remover. Then use the pliers to untwist the two wires holding the ribs to the plastic parts on the stem of the umbrella. Unthread the wires from the ribs, reserving wires for later use.

The cloth part of the umbrella with the ribs attached should be separate from the stem now. Clip off two ribs opposite each other, and save them for ear frames. Split the umbrella in half down the seam previously occupied by those ribs. Using tin snips (or other cutters), cut off extraneous rib parts so that the ribs will be slightly longer than the fabric when they are joined together. If you have pointy bits sticking out, you can tape them down.

Pin the umbrella halves to the sleeves and sides of the sweatshirt, starting at the armpits. Pinning is a good idea since the umbrella nylon is slippery and the sweatshirt stretches. Sew the umbrella on, leaving a gap in the armpit for the rib ends to poke through.
Trim off the extra flaps of material straight across from the bottom of the sweatshirt to just before the point where the bottom rib is attached. You'll need these triangles of material for the ears.

Here's the gap where the ribs stick through. This will give you some flexibility of movement when you're wearing it.

Use the wires that were holding the ribs together (or any other convenient wires, string or thread) to tie the three rib ends together.

Take the two extra ribs and trim extra points off so that you can make a (very uneven) tripod out of them. Stitch the point of the tripod to the point of the triangle. Stitch the long and short ends down as well. Bisect the angle with the middle piece and stitch that end down, too.

You'll need someone (or a wig frame or mannequin head) to help you with this part. Getting the tripods into just the right triangle for ears may require repeated tries. My personal voodoo doll yelped several times as I adjusted them. The key is that the middle one needs to be set quite far back. The outer two need to be approximately in line.

After getting them nicely lined up, stitch them on and then trim off the excess fabric.

Your top is ready, serve with matching shorts and/or miniskirt (or alternate dress appropriate to your climate) and you are now ready for trick-or-treating.