Planning is a smart way to start making your costume.  Tips for good prep work are below. Blank templates Fabric pen Paper bags, model, or mannequin Foam board
Step 1: Though nylon/lycra doesn't need to be shrunk like cotton to find the final fit of the fabric, you'll want to wash your unitard before beginning any work on it.  Chemicals referred to as sizing are present in most new fabrics, and they can cause skin irritation and interfere with how well your unitard takes to paint and dye. An product called Synthrapol can be used during the first wash of fabric to remove any invisible dirt, oil, and chemicals that may cause uneven coloring when dying or painting.  It can also be used in an afterwash to prevent loose color particles from re-settling onto the fabric in unwanted areas.  When washing your unitard alone, use approximately 1/8 cup.
Step 2:  While your unitard is on your body, use a mirror or have a helper map out areas of color.   If you are using a mannequin, you will be able to do this yourself.  Map your areas slightly larger than where you want your color to end.  If your color is dark, don't worry about erasing the pen; simply color over it.  If you will be using a light color, erase the pen with a damp cloth just before you begin painting.  The damp area will outline where your lines were, guiding you as your paint. Note: I strongly recommend buying a fabric pen instead of a fabric pencil.  I use a wonderful pen that has two tips: purple at one end, and blue at the other. The purple will disappear all by itself in a matter of hours, and is a wonderful temporary guide while you are painting.  The blue requires water to disappear, and I do not use this quite as much because I have found that if left on for several weeks, it becomes semi-permanent.  However, it does have it's uses from time to time, and having both options has made my costume mapping a breeze.
My favorite product for hatching (linework) is a bottle that can be purchased through Dharma's Trading Company, #7 and #9 tips.  I have tried other bottles, including what you can purchase at AC Moore/Michaels, and have not been happy with them.  The Dharma bottles are worth the investment as they will give you beautiful, controlled lines.  The only downside is that you must refill them about a third through your unitard (per layer) but to me the result I achieve is worth it.  (I personally fill the bottle 2/3 with uncut (not watered down) paint, fill the rest with water, put the lid on, shake it very well, and have the perfect consistancy, which means less mess pre-mixing your paint in a separate bowl!)
It is also important to realize that pencil-straight lines will make your costume look homemade and amaturish.  The more movement you put into your paint strokes, the more real your costume will look.   Don't be afraid to design with curvy lines, especially around the legs and arms.   My favorite way to achieve this without being messy is to use the blue fabric pen mentioned above.  I draw my "curves" about 4-6 inches apart so it looks like a paint-by-number canvas.  This will keep your lines nice and uniform and make painting a breeze so that you won't be trying to design while you paint.
Step 3: There are several ways to set up your unitard for painting. 1. Working FLAT with paper bags or cardboard cut to fit inside the unitard will prevent paint from seeping through to the reverse side of the fabric and can be easily adjusted depending on the section of fabric you are working on. Place an old sheet or towel beneath your costume to prevent your work surface from becoming stained.  Never, ever use newspaper, as the black ink will transfer onto your fabric. 2. If you can find a willing model, painting your unitard THREE-DIMENTIONALLY on a person can be a wonderful way to see your progress as you go along.   3. Mannequins, which run about $100 - $200 (shipping included) on eBay, is another THREE-DIMENTIONAL option that is too expensive for many people.
I personally prefer to work flat when hatching (for good control) and 3-D for base colors, gradients, and other designs.  The best way to work flat is with a large piece of foamboard and thumbtacks.  Stretch your working-portion of the unitard as tight as it will go and hold it in place with thumbtacks; they will not damage the fabric.  This will prevent paint from bleeding and rubbing onto other parts of the costume.
Step 54: Lay out your supplies before you actually begin painting. This will help your progress go smoothly with less mistakes. Photo to the right: my handy-dandy supply box, which includes materials for all aspects of costuming: studs, spikes, rotary punch, brushes, squeeze bottles, Jacquard Airfix, wig pins, wig cap, scissors, superglue, fabric pens, needles, snaps, and other such materials.
If you plan to paint your costume on a person, be sure they wear old underwear. The paint will bleed onto their skin and stain whatever is worn underneath.  Paint will usually come off the skin completely after two or three showers. If you purchase a mannequin, consider wrapping him or her in saran-wrap if you would like to resell it in the future. Unless it is sold to a fellow costumer, most stores or eBay buyers will not appreciate a paint-stained form.