I'm often asked how I get my stitches so even, and I have two answers:
1. I'm just really, really good at stitching
2. I make patterns for everything
I measure all my stitches out by hand with a pencil and a ruler, and then scan the illustration and use Photoshop to place little red dots for each stitch. The image is then cleaned up, and I have a pattern I can print out and use over and over. Creating the patterns is the most time-consuming part of my process.
The pattern is laid over the paper image, and I use a needle awl to poke all the holes for stitching (last year I got a 'cushion grip' designed for crochet hooks; it has saved my fingers from the blisters that used to accompany all that poking).
This video shows the process from poking the holes through to the stitching. My paper pattern piece ends up in pieces by the end, since I cut away the parts I've stitched and use the remainder to make sure I get my stitches right. With each hedgehog spike made using three holes and two stitches, it's easy to get lost and accidentally place a stitch in the wrong place.