Step 1: What you will need:
Fabric. I used two contrasting colours of velvet because I didn't have enough of the green for the tail as well. I did have to be extra careful when cutting out because of the nap. Fleece would work just as well.
Interfacing. I think I used around 1 metre for an adult sized costume. This stabilized the velvet and made it stiffer so the spikes stood on their own.
Stuffing, enough to fill the tail.
Onesie (I bought one to save time but feel free to make one if you have a pattern lying around)
Paper scissors for cutting out the interfacing because I didn't want the glue on it to mess up my fabric scissors
Fabric scissors for cutting out the velvet.
Triangle templates for the tail and the spikes. Don't forget to add seam allowances.