1. Come up with a budget. What you may think is an inexpensive and simple costume will often times blow your budget out of the water.
2. Based upon on your budget choose a character (does not have to be gender based but helpful if you are trying to keep it real)
3. Collect as many reference pictures as you can of the character you want to costume. This is very helpful when you start looking at materials, props, etc. It will also help you determine number 4.
4. Choose whether you want to be screen accurate or "close enough." This will determine what you can use, buy, or make yourself. Remember that screen accurate tends to be more expensive than "close enough."
5. Once you pick the costume figure out what you can make vs. purchase. Making small parts of the costume is always fun and very gratifying. Purchasing other parts of the costume is fine especially depending upon your talents. Don't overspend though because you can kill your budget instantly.
6. Dig around in your closet, basement, garage, etc. and figure out if you already have parts and pieces to your costume already. That way all you have to do is modify those parts and pieces.
7. Start searching the web for people that have already completed the costume because they will be a cornucopia of help. Those people will have alot of helpful hints, tips, shortcuts, and cost saving techniques (unless they are going after screen accurate which means they may not have a budget.)
8. And finally...post your progress on your own thread here on RPF because watching a costume develop is fun to see and it also provides people to comment and provide tips, tricks, and secrets.
Good luck!!!!:love
Added: Check out some of the websites that are specific to the character you want to costume. Some great costuming websites for general information:
www.cosplay.com