Look at the colours when they're printed out! Printers can interpret colours differently. Compare them to the images on the computer and write notes on your print-outs if you need to remember how they differ: "actual costume about a shade lighter" or "a little less yellow, a little more red" are simple notes that will work for you when you're shopping.
Put the print-outs in a folder to keep them neat and clean. You will use them a lot and you might as well not waste the ink printing them several times.
• Make an itemized list of the costume parts. Look at a full body shot of your character, or a few images that together show the whole costume. Go literally from head to toe and write a list of each component. For example, with my Souseiseki costume, I wrote a list that read: hat, wig, cape, shirt, vest, pants, stockings, shoes, scissors (prop).
Then, go back to each item and really break it down. For example, my first item is the hat. Am I going to make the hat from scratch? Buy it? What is it made out of? I decided I'll probably end up making the hat, so I listed "cardboard, black fabric, blue fabric". Do this for every item. Think about all the little things.

For my shirt, for example, I'll obviously need white fabric, ruffles (or more white fabric to make ruffles) and buttons for the sleeves. But I'll also need something to close the shirt, either buttons or a zipper, as well as interfacing to keep the cuffs in shape. These are all things that are easy to forget in the moment when you're shopping, so really take the time now to think about your costume in as much detail as you can fathom. The fewer trips you have to take, the less stressed you will be.
• Fill in the blanks. There will sometimes (okay, often) be bits and pieces of the costume that you aren't able to find reference material for, try though you might. It becomes your job to fill in the blanks and decide what to put in the unknown areas. Common missing links are the back of a costume and the bottom of the costume (shoes, etc). Look at the rest of the costume and try to think about what makes sense here. If you're scared to make something up, go with the simplest solution: mimicking existing elements. Don't know what the back looks like? It's possible it looks the same as the front, within reason. Or, it could be simply plain. Try to keep within the character theme. Please, DON'T use this as an excuse to wear your big shiny platform boots just because the shoes are an unknown. Think about the character and the costume. What goes with it? What would he/she wear? Do your best!
• Turn your list into two usable things: your task checklist and your shopping list. First, your task checklist is a big master list you can carry with you or post on the wall of every tiny thing that must be done. Don't gloss over anything! That can lead to BAD STUFF such as: forgetting to do something, thinking you don't have as much left to do as you actually have, skipping a step and having to redo something, etc. Make headings for each costume component and start listing them. For example, for Souseiseki's cape, I have the following:
- Purchase materials
- Sew ruffles
- Sew collar
- Sew cape
- Attach collar to cape
- Attach ribbon
These are all little steps that add up to creating the overall cape. Each one gets a checkbox next to it and doesn't get checked off until that part is TOTALLY complete.
Breaking this part into little pieces also helps you feel like you're making progress (which you are!) because you can usually check off a few of these little things each day.
The second part of this step is the shopping list, which is pretty self-explanitory. Take all the little things you wrote down and make them into a practical shopping list to take along. Divide the items up by store (craft store, fabric store, hardware store, online, etc), and remember to include quantities!
We'll talk more about the shopping part next time, but for now, I hope these steps can help you to clear your head and feel super-organized going into your project. I've found taking the time to do these things at the beginning helps me feel less stressed and allows me to have FUN making my costume... don't forget, that's what cosplay's all about!