The Mardi Gras mask is a half mask: it covers the eyes only, leaving the nose and mouth exposed. It is a generic mask with no particular attributes and, as such, it is one of the most versatile masks in this book.
The Monster Mask is easy to make and very accommodating. Since no one really knows how a monster looks like, you can make it any way that you like and it will still be a monster (long or short ears, wide or narrow eyes etc).
The cat mask is the most difficult. It requires two pieces of paper (one for the head and one for the whiskers). Note that the eyes on this model are round: it is actually somewhat difficult to cut a perfect half circle, so you may need to make this mask a few times before it is perfect. Unlike monsters, everyone knows how a cat looks like. Thus, any variations on the folds will change the facial features causing it to look (or not look) like a cat.
The Skull Mask is getting a little more difficult to fold since the model is slightly curved and has a bit of a 3D effect.
The Daruma Mask is easy to fold but cannot be worn. In Buddhism, the Daruma Mask is used for making wishes. Make a wish and then draw in the pupils in Daruma's eye. Make another wish and draw in the second pupil. In this mask, you can fold the paper to make the pupil instead of drawing it in.
The Elf Mask is delightful. The folding sequence is relatively easy, but cutting the eye and mouth holes is difficult. The best way to make the eye holes is to make slits in the paper with an exacto-knife or a very sharp knife. These implements are very sharp and can be dangerous; parental help is advised. This mask can be worn in front of the face, but it also looks great in front of a greeting card.