Here's some pic loaded goodness below. I'm like a 5 year old how I rely on pictures for instructions, hope you all feel the same way.
Materials: Foam sheets (craft store), scrap fabric, fabric to cover the spike (any tight weave), glue gun, sewing machine with thread.

You might want to keep in mind where you want to put the spikes later (the possibilities are endless!!) so you know how big you want to make them. Cut out a circle from a foam sheet with the radius of how tall you want the spike to be. You can use a cup or other circular object to trace the shape on the foam. I did it free hand, so really it doesn't matter if it's not perfect.
Now you want to cut a slit in the circle. The slit should cut to the middle of the circle. To make sure you're getting the right spot, try lightly bending the circle in half then cutting along the fold (half way of course).

Now you can play around a bit to see how wide you want the spike to be. You can make wider, shorter spikes or skinny pointy ones.
Grab your heated up glue gun and make a straight glue line where you want the spike to end. Quickly roll the foam over and hold it until is cools a little bit. Carefull not to burn yourself, not that I did or anything...

Woopwoop, you got your cone/spike shape! If the base is not even and doesn't stand flat you can trim it so it's even all around. This happens when you don't measure your circle, but I'd rather just eye-ball it and make a slightly larger cone in case I need to trim.

Cut or shred little pieces of the scrap fabric to stuff the spike. This makes it stiff and keep it's shape. The reason I didn't stuff it with batting/stuffing was because I thought fabric would be a stiffer hold, I don't want the spike to be too spongy.
Use a pencil to stuff the little pieces all the way to the bottom. Stuff it tight full, but no so much that the spike is bulgy and bumpy.
Get your glue gun again and glue all over the top of the stuffing and rim of the spike. Quickly press it down on some strap foam you have.
Once it's cooled, trim carefully around the base and the cone is done. Now we want to cover it with fabric to make it extra fancy...
Cut a circle in your cover fabric that's at least half an inch larger than the original circle used for the spike.
Arrange the spike in the fabric so it covers it all the way around and there's no hole poking through the top. Use a pin to show where you're going to want to sew.

Pull the cone out and head over to your sewing machine. Sew a straight stitch (with matching thread) where you marked with the pin, making sure you go all the way to the end otherwise it will have a little hole on the tip. You could also use chalk to mark out where you want to sew but because, again, I did a lot of guess work I had to re-do a few of these fabric covers.
Okay, once you've got the cover, trim the edges, flip it inside-out an slip the cone in. Make sure it fits in and not baggy. If it looks good, get your glue gun again and glue a little bit around the base of the cone then CAREFULLY press down the fabric edges on the glue. Wait about 10-15 seconds so the glue cools a little and you don't burn yourself. Use a pencil or other random object the press it down if you don't want to risk burning your poor fingers.

I decided to spike up some black stirrup leggings. How hardcore is that? Picture them, just jutting out of my legs like a dinosaur. I'm sure to frighten all in my path with my spiky wonders, muaahaha.