Starting to cosplay or costume can be intimidating. It's really not that bad, though, when you break it down into steps. I'll show you what I do.
First, pick a costume. I've decided on Tsukimi from Princess Jellyfish for this example. I chose it because I love the series, I love the character, and I've got friends who want to cosplay the series with me!
Second, choose a time frame. I want this costume for this weekend's Kansas City anime con, Naka-Kon!
Next, start to research the character and choose a costume. Many characters have more than one outfit, so pick the one you like best or think you can do the best. I'm going with the iconic, easy one:
Next research the costume. Start with Google Image search. Often with anime or drawn media you can find a character study, like this, which is great because it gives you all angles:
Next, using the images you found make a list of all the parts of the outfit. Sometimes this involves guesswork and deciding "Is that really an underskirt or just a fake split?" If you have to you can research more into the time period or traditional dress of the source to help educate these decisions.
After you've got a list go back and start thinking about each piece individually. Is that part something you can buy or will you need to make or modify it? If you're buying it, from where? If you're making it, what pattern will you use? What kind of fabric? (We'll cover these in more detail later, just know you need to think on them.) Notate all this on your list.
Pants - buy from WalMart, add pocket
Shirt - buy from WalMart
Socks - buy from WalMart
Sandals - buy from WalMart, add strap
Black Wig - buy from Epic Cosplay and style
Glasses - buy? KC Costume
Brown Contacts - ignore, don't want to bother
Once you've got a list of pieces start to break down each piece into steps to complete. Sometimes this can be complicated, sometimes it's easy.
Add pocket to pants
Add strap to sandals
Buy red nail polish
Paint glasses red
These lists can get really long and complicated for a big costume, but breaking down the list by piece can really help you to think about what you need to do for each step. Now is a great time to figure out what you do and don't know. Need to figure out a way to add a strap to sandals? Google tutorials, ask friends, look for help. Figure out what steps you're going to need help on and try to figure out where to get that help.
Next, look critically at your list of steps and the time you have between now and the convention. Do you have time to complete all the steps and not go insane? Be sure and add in time for testing steps like mocking patterns and trying out new techniques. Also factor in shipping times. It's a good idea to add deadlines to all your steps at this point. I like to use ToDoIst to chart my schedules and provide me with a daily list of what to accomplish to help keep me on track. I used to use a notepad of sticky notes where I'd just write down what I had to accomplish and then tear off each day. Use what works for you. Look critically at your calendar while scheduling. Do you have a family event that will not let you sew that weekend? Move things to the next weekend. Also think about how long each step will take you and plan in extra time for mistakes.
Once you've got your to-do list you're ready to start! Order your supplies and get going!