First, I'd make a circle, and mark the center and where they eyes go with some lines.
Second, I'd add the ears. Depending on the breed, I would shape them differently.
Next I'd add the muzzle shape. I usually shape it differently if the cat is more muscled. Like, if I were making a fat cat, it'd be roundish, and if they were more muscular, and make it more of a square shape.
Now I add the sides. As I said before, the shape and size really depends on the breed you're drawing.