Learning how to make a Whoville costume can be fun and creative, because the costume can be pretty much anything. You don't really need to make the actual clothing items, but rather put together a bunch of things you already have or find at a thrift store to create a great Whoville costume.
In the illustrations in the classic Dr. Seuss books, the Whos don't really wear many clothes. They are really just furry little things sporting collars and big ties. Most of the female Whos are drawn with flowers on stems growing out of their heads.
The 1966 cartoon movie version of "How the Grinch Stole Christmas" is fairly similar to the books, except they are wearing clothing. Each Who has a different color and they each dress head to toe in that color.
In the more recent movie adaptation of "How the Grinch Stole Christmas," the clothes are much closer to what normal people would wear, which could make the Whoville costume a little boring. This movie does do the Whos' hair in a very interesting way, however. The Whos have what may be called "Crazy Hair," but a very important thing to note is that "crazy" does not mean messy. The Whos sport unusual braids and hair piled on the very top of their head, but it is always done very neatly and precisely.
In the stage musical Seussical is where you will find the most exciting and interesting Whoville costumes. The Whos usually wear all sorts of crazy patterns and colors. For example, one Whoville costume might be a plaid jacket of one color, polka dot shorts of another color, and boldly striped tights or socks.
For your Whoville costume, I would recommend choosing to do either all one color or all different bright colors on one costume. Bold patterns are a must, such as plaid, stripes, and polka dots. The bigger and bolder the pattern, the better, and don't be afraid to combine ones that don't match. If wearing pants, crop them short and wear striped tights or socks underneath. Boys can spike their hair up high and color it a bright color. Girls can do braids or pony tails in weird places, but make sure it's done neatly. Flowers in the hair are a great touch. You can use fake flowers, or you can make cute flowers with craft foam and pipe cleaners for the stem so that they stick out from your head.