You'll need about a quarter yard of fabric; an eighth of fur or velvet or something else soft, and an eighth of outer fabric. These can be the same but I like contrast. I use scraps from other projects.
Measure your head at its largest circumference. We will call this measurement H. From each of your two fabrics, cut a rectangle that is H plus 1.5 inches long, and 6 or 7 inches wide (depending on how big your head is). These will be the sides of the hat.
For the crown, cut an oval shape that's H + 2 inches around, or thereabouts (don't go much smaller than this). I made a paper pattern by folding a piece of paper in quarters, then cutting a line about (H + 2) / 4 inches long (see pic). Cut this out of your outer fabric.