Step 1: The Check List
Step 2: Contacts
I have found solid coloured lenses to be very striking, but since they only have a small pupil opening, your pupil always seems small (not always a bad thing) and the outer edges of your 'vision field' can be a bit blurry. I have yellow, red and cat-eye ones I use for creepy, unnatural costumes. I have worn contacts made by Acuvue, Geo Medical (Crazy Lens and Super Magic Color) and G&G (Angel line).
'Enhancer' lenses can be worn by people with light eyes. I naturally have grey eyes, so when I put in blue or green enhanced lenses, my eyes take on a natural hue (sometimes the colour is very vibrant depending on the light, but it still looks 'real'). I wear these on a regular basis, and also when I dress as characters with bright eyes, like Katara or Aisling.
Circle lenses make your iris look bigger and give your eyes a doll-like effect. I use them when I want to look a bit more doll-like (like for Björk) or somewhat freaky (like black ones for a sorceress).
You can find prescription and non-prescription contact lenses for a wide range of prices online, but my advice is to consult an optometrist before you purchase and wear contacts. Since I know some people just aren't going to do that, at least look at contact reviews online from past customers of lenses. At the bare minimum, PLEASE, wash your hands extremely well before handling your contacts, *clean* your lenses before wearing (some you may have to soak in a solution for up to 8 hours before putting them in) and if you find them to be irritating or uncomfortable or you can't see at all...take them out!!! Not everybody is able to wear contacts comfortably, and if they are bugging you, they could be causing damage to your eyes. If you can't wear them, the world will understand.
Putting in and taking out contacts does take practice; I've been doing it for over 14 years, so it's a very natural process for me and I've likely developed some habits that don't work for other people. But in general what you do is (after washing and drying your hands) you remove one lens from its case and sit it on the tip of your finger - making sure it is sitting 'right side up.' The contact should look like a rounded little bowl (otherwise it's inside out. If you put it in inside out, you'll feel it!). Looking up, touch the contact onto your eye, remove your finger and (slowly) blink. Some people use their other fingers or hands to pull up/down their eyelids to help get their contact in. If you haven't worn contacts before, it likely will take some practice. Don't give up - unless you find that your contacts are hurting you!
Step 3: Wigs
Finding and buying a good-looking wig is the first step. I buy all of my wigs online, going off of customer reviews, so here are a few of the places I've bought various wigs from.
Jenny Hair Sense
Before you wear a wig, you need to put on a wig cap (basically a nylon-head-sock). And before you do that, you may need to put your hair up. You need to pin your hair in place (using bobby/bobbi pins) and pin your wig cap and wig in place (using hair pins).
What I do is roll my hair up into at least seven pin curls - these are sections of hair you wrap around your finger then lay flat against your head, pinning them into place with bobby pins.You want five pin curls around the crown of your head (one in the middle, two on either side) and then two in the back. Pin curls are great because they lay flat against your head, and if done properly, won't hurt (like wearing some braids for an extended period of time do). Pin curl or french braid the remaining hair on your head.
After my hair is pin curled into place, I put on a wig cap. Stretch your wig cap out with your hands and place it over your head, having it go well past your eyebrows. Pull back gently on the cap until its sitting along your hairline. Hopefully this will collect and keep in place any loose hairs. You then pin the wig cap into place using hair pins - sometimes I just drive the hair pin directly into a pin curl (through the wig cap). Other times, when I need a wig to be a bit more secure, I start by catching the pin curl with a forward-facing (not towards the back of your head) pin, then turning the pin down and back, embedding into the pin curl. I do this at least five times, for all of the pin curls along the front of my head. At the back, I criss-cross two larger wig pins into place.
Finally comes the wig! I position the wig along my eyebrows, then pull it back into place. Using the wig pins, I repeat the pinning method used for pinning the wig cap. I've gone running, spent all day in the sun, gallivanting, and sword-fighting, with a wig, and have yet to loose one using these methods.
For a more in-depth look at how to put on a wig, check out taeliac's tutorial.
Step 4: Eyebrows and Eyelashes
To conceal your eyebrows, go over your brows with a glue stick (apply in the direction of hair growth). As it starts to set, add normal-skin foundation. Set this with powder when it dries, then apply any additional makeup/concealer over top!
At this point, you can draw on eyebrows with an eyebrow pencil (some people use eyeliner, some even use acrylic ink!). You can also put on artificial eyebrows. Once I cover my eyebrows with a glue stick, I apply false eyebrows by putting a thin layer of spirit gum to the back of the fake eyebrow, then sticking them over my real brows once the spirit gum has turned tacky. I have also used Pros-Adie. I apply Pros-Aide to both the back of the false eyebrows and over my real (covered) eyebrows, let it set for about 20 seconds, then apply the brows.
You can temporarily colour your eyebrows by coating your natural brows with white glue (using a q-tip or fine paintbrush). After the glue dries, you can apply acrylic paint or eyeshadow over top. Afterwards, you can wash out the colour and glue with warm water and soap.
Eyelashes are also a good addition to a character. You can buy a variety of eyelash strips that come with glue and tweezers and instructions on application. I like to use individual lashes/groups of three lashes to fill out my eyelashes a bit more naturally. They are applied in the same manner - by adding a bit of glue on the eyes and placing them along your eyelash line with tweezers. Curl your false and real eyelashes together with an eyelash curler, add some mascara, and you're ready to bat your eyes.
Step 5: Prosthetics
I make my own prosthetics using slush latex (which is like really thick liquid latex). Foam latex pieces are lighter and more flexible (but beyond my means to create). To make my own pieces, I sculpt (in this case) a nose onto a copy of my face, make a mold of the sculpt, then cast the nose in latex.
I use sulfur-free oil-based plasticine to sculpt. Once I have finished, I apply a release agent (such as Vaseline) over the sculpt, then apply layers of Ultra-Cal 30 and cheesecloth. Once this Ultra-Cal mold has cured, I clean out the clay sculpt from it, and pour slush latex. I paint the latex around and scoop out the excess - you only want a thin layer. After the latex has dried, I apply a bit of baby-powder to the inside of the nose (to prevent the prosthetic from sticking to itself) then remove it. For a more detailed tutorial on sculpting, casting and molding with these materials, check out my Asari Instructables.
To apply the prosthetic to my skin, I use either spirit gum or Pros-Aide. Apply some spirit gum along the back-edge of your prosthetic and wait for it to get tacky (about 20-30 seconds) then apply to your skin. For Pros-Aide, you apply the adhesive to both the prosthetic and skin, let it set for about 20-30 seconds, then put on the appliance.
To colour the latex prosthetic, you can use rubber mask grease makeup, or you can use creme makeup if you prep your appliance first. stipple the latex piece with castor sealer. Blot, then let the sealer dry a bit. Set the sealer with powder, then apply regular (creme) makeup (which you also powder afterwards to set into place).
Step 6: Zits and Boils
All you need is a little bit of gelatin, a little bit of water, a little bit of flour, and some icing tips (#1-3). You mix all of those three ingredients (you don't need much) until you get a runny paste-like consistency. You may have to add a little bit more flour or gelatin - just remember to make a very small amount. Once you have your paste, spoon a little bit of it into an icing tip. Using your finger, squeeze the paste out and dot your face (or neck, or arms). The mixture will dry fairly fast (that's why you don't want to make too much of it).
After it's dry, you can apply a variety of makeup to help make your zits and boils extra-delightful. I like using creme makeup, applied with a small brush or q-tip, and I set everything with powder and a powder-puff afterwards.
Step 7: Scars, Blood, and Bruises
First apply makeup to your skin where you want the scar to be. This could be pink, red, white, brown, or a mixture, applied in a line or jagged shape (any shape you want your scar to be). Paint over-top on a layer of rigid collodion. Let it dry. Then apply another layer. Let it dry. Repeat this for eight more times, and your skin will pucker and look like a scar! You can apply a bit more makeup if you like. A little bit of dried, day-old fake blood can go a long way.
To remove, carefully peel the scar away - but know that when you remove it, you'll also be removing a thin layer of skin along with it. You won't notice this, unless you then reapply another scar in the same place right after - then you might actually get a real scar :S wait at least 3 days before reapplying another scar in the same spot.
Sometimes I follow the scar procedure, then fill in the scar area with a bit of blood, to represent a cut. And sometimes I apply some bruise makeup to it too.
Bruises, whether they are subtle or attention-drawing, can be achieved with makeup. I like using the Ben Nye creme wheels, which I apply in various colour stages and then powder to set. The severity and intensity of a bruise can be achieved from apply yellow, green, red, blue, purple and brown makeup.
Step 8: Teeth
I like to use tooth enamel, to slightly (or dramatically) alter the colour of my teeth. Simply coat dry teeth with coloured tooth enamel, wait a few seconds for the colour to dry, then go about your day. Remove the enamel with a thorough cleaning, with your regular toothbrush and toothpaste!
False teeth are an awesome consideration - single fangs, crooked, straight and/or aged veneers are great! I find them a bit uncomfortable, but if you apply them correctly (using denture adhesive supplied when you purchase your teeth) they can stay on for extended periods of time and really add to your character.
Step 9: Put It All Together
For me, it's helped make my costumes a little more magical - and in this case, a little more icky; family and friends have a hard time looking at me when I'm Tatterhood!