From full masks on regular Hollows to partial masks on Arrancar, this tutorial can help you on all fronts. I'll be using photos of my Ichigo mask as a reference point.
-1 bag of paperclay (~$5.00)
-1 plastic mask (~$3.00)
-2 packs of fake fingernails (french style with squared tip is best) (~$5.00 ea)
-Gesso (or a lot of patience with regular white acrylic paint) ($6.00 sm bottle)
-Red paint (~$0.88)
-Paint brushes (~$1.00 per large and small soft-bristle brush)
-Rough and soft sandpaper (varies)
-Scissors (you should have these already)
-E6000 craft glue (you could probably get away with hot glue) (~$4.00)
-2-3 nights (I worked on it in conjunction with styling my wig and checking the boards)
STEP ONE) Construction - Image>>
-Cut out any non-stabilizing sections of the mask (I removed the back of the jaw/cheek area on mine, but left the sides of the eyes for now.)
-Use the Paperclay to begin creating the profile of the mask. I started with the side because the nose and chin are very defined, and then worked my way to the front-profile for the ridges around the eyes and the figure of the outside edges of the mask. Don't make the clay too thick otherwise it'll be needlessly heavy, so only bother with showing thickness around parts of the mask that can easily be seen when it's worn (ideally, just around the eyes and jaw-line, pending the mask you're making.)
-Let the mask dry OVERNIGHT. Because of how thick the paperclay is around the middle, sanding or tampering with it too early will mean the moist clay under the surface will warp and thus make the rest of the thing look funny.
STEP TWO) Refinery - Image>>
-Sand the mask. Sand it good. Also, this is the time where you want to cut out any pieces of the mask that you don't need (such as the sides of the eyes/jaw or under the chin like in mine. Leaving them in-tact until this point helps to retain the shape of the mask while under the weight of drying paperclay. If you cut out those parts at the beginning then it's possible the mask's shape will have changed and won't fit on your face as well as it could.)
STEP THREE) Painting and Repairs Part 1 - Image>>
-I used the Gesso all over the mask at this point. Gesso is great because it's an EXTREMELY heavily pigmented white acrylic paint so it'll help to fill in cracks and subtle variations in texture. I put about 7-10 layers on it because I had a few deep cracks come up over the bridge of the nose, but you can decide when you've got enough.
-Use the E6000 to glue together any MAJOR and significant cracks after you've applied the Gesso. The heavy, thick layers of paint will help the glue hold the crack together whereas the clay would just re-break again if glued without the paint.
STEP FOUR) Arranging Your Teeth - Image>>
-The fake fingernails I bought were simply the cheapest I could find, and they had 24 nails of different sizes. I generally discarded the pinky-finger ones because they were way too small. One pack should cover the top row of teeth and the other should cover the bottom. You'll need 7 teeth on each side/row (for Ichigo's at least) and the top-row teeth should be slightly rounded at the corners. The root-area of the tooth should be cut into a rounded-point, as per your references.
-I labeled my rows of teeth as such:
7 6 5 4 3 2 1 1 2 3 4 5 6 7
G F E D C B A A B C D E F G
There were dots as well on the teeth for the right, and NO dots on the teeth for the left.
-Use the E6000 to glue each of the BOTTOM ROW teeth to the mask first. Make sure you start from the center and work your way to the sides. Once the glue is dry (you can work over it in about an hour, but it takes 24 to cure) I'd apply several layers of Gesso. It'll help hide the application points and smooth the contact-point between the tooth root and the mask. Apply the top-row of teeth after that in the same manner.
STEP FIVE) Final Touches - Image>>
-When you've got enough Gesso on there that you can't see the numbering-marks on the teeth, get out your red paint. Use pencil very lightly to mark off the spots you want to color in and then very carefully start to paint.
NOTES ON WEARING IT:
Facials: I'm going to be attaching two eye-and-hook deals into the forehead of my mask so I can attach it directly to my wig and be able to remove it easily. I'll probably also use some double-sided medical tape inside the nose to hold it onto my face if I intend to wear it for long periods of time without removal.
Partials: You can either use medical tape or even spirit gum to attach it to your face. DO NOT USE DUCT TAPE, MASKING TAPE OR ANY OTHER KIND OF GLUE, BECAUSE IT CAN AND WILL TAKE SKIN WITH IT WHEN YOU REMOVE THE MASK.
NOTES ON OTHER MASKS:
Partials: You don't need to buy a plastic mask for it, but I recommend buying at least something that resembles the shape of the part of the face you'll be covering so you have a base to work on.
Helmets: It'd require a whole new tutorial to go over these, such as what Ulquiorra wears, but suffice it to say, you can use fun foam and paper mache to make the initial base for the mask and then use the paperclay method to finish it.
Hope this helps! Woo Bleach!