I just finished the first half of P's Halloween costume; it was a blast to make! And he LOVES it
All I needed was a cheap hooded sweatshirt and some felt
The spikes on the back are slightly bigger than 3" on all sides, the ones on his head are 2.5". I cut two triangles for each spike and sew them together with a simple straight stitch on each side.
To attach the spikes I cut down the seam in the hood and down the middle of the back of the sweater. Then all you have to do is turn the sweater inside out, right sides together, sandwich the spikes between the two layers and sew straight down the hood and back.
And you're done!