DO NOT START WITH THE HEAD PIECE.
Its going to be your BIGGEST pain in the ass. And you will get upset and want to quit if you start with that. Get the simple things out of the way first. Another reason for not doing the head piece first is that you can still wear your costume with out it. So you can still go to cons and have fun.
1. Draw out your design
2. Don't look at the whole thing, look at what makes up the whole. It will be less daunting that way. 1 piece at a time.
3. Find your pattern(s) Simplicity Pattern #5840 is the best "Jedi" pattern I have found
http://www.simplicity.com/p-2272-misses-men-teen-costumes.aspx (though it doesn't come with a pants pattern but I would just Goodwill those) For a skirt I like Simplicity Pattern #5914
www.simplicity.com/p-2276-misses-skirts.aspx (skirt D)
4. Start with your tunic, obi (the thing that goes under your leather belt), and pants or skirt. These are pretty easy for first timers.
5. Robes are ALOT of fabric. So they are going to be one of the last things you do.
6. Have someone help you. You are going to need someone to pin the hem of your robe to you and your skirt (if you go the skirt route)
~TIPS~
~FALLOW the pattern. READ and take your time. (trust me the people who write these things can't write to save their life)
~DON'T RUSH. You will make less mistakes and your costume will look ten times better.
~Iron your seams. This makes your life easier, and makes your costume look nicer.
~For Simplicity Pattern #5840 Robe B you only need about 5.5 yards of fabric. Don't spend all that money on those extra yards if you don't need them fabric isn't cheep.
~Finish your edges. Use shirt tail hems and zig-zag stitch when you need to finish an edge.
~Small stitches are better your fabric will hold together longer. Though can be a bigger pain to rip out if you mess up. Another reason to take it slow.
~Use the right kinda of thread. You make your stuff in 100% cotton, use cotton thread.
~If the pattern calls for 4 yards get 4.5 yards. You will make mistakes and need that extra when you are first starting.
If you need help ask, I try and check back here daily so you can shoot me a PM if you need help. I know I just threw a lot of information at you.