I admit, I'm confused as to what you are confused about, since you say you have sewing experience and people to help you, but I'll take a stab at this.
1. Look at the costume. Really look at it. Get multiple reference pictures that show front, back, and sides, standing still and in motion.
2. Break the costume down into its component parts. Costumes, with a few exceptions, generally have both soft parts and hard parts - this has pants, a shirt, a jacket, some armor, and a prop sword.
3. Research. If you know how to sew, you likely won't need a sewing tutorial. If you have never made armor before, or have never used a pattern before, there are tutorials on this site and also searchable via Google. Read up on a LOT of different techniques for making the armor - how you do this will depend on your budget, timetable, labor involved, skill level, accessibility of resources (Wonderflex is easy to get, but most people don't have access to a vacu-forming table), and your comfort level with the techniques described.
4. Decide what you will make and what will be store-bought. The black pants and white T-shirt, for example, would be much more easily, quickl, and cheaply procured by just buying them in a store rather than making them. You may not find an accurate jacket, so this may be a piece you wish to make.
5. Identify what you will need for the items you decide to make. The jacket will need an outer black fabric, a white material for the lining, and what appears to be a chunky separating zipper down the front. Of course, you'll also need thread, and probably interfacing, and perhaps a pattern, unless you intend to draft your own. Read through materials lists on armor tutorials.
6. Go shopping! Using the list you just created, get your materials. You don't need to buy everything all at once, but make sure you have everything you need to tackle the first several steps of your construction.
7. Figure out an order to approach the project. All armor first? All soft parts first? Get armor started and then work on soft parts while the paint cures on the armor?
8. Start working. You will be the best judge of what methods you will use and how well you work on your own, but try to figure out a schedule so that you're not attempting to do the bulk of the work the night before the convention.
Hope this helps!
NOTE: Also, much of the weight of the costume will depend on the type of material you choose. You can select a lighter fabric, but keep in mnd that jackets are by nature designed to be a heavier weight. Fabric weight is extremely important when it comes to making a garment - using an unsuitably light material, no matter how well sewn, will make the garment look cheap and flimsy.
Pictures and tutorials from my costuming portfolio at Syagria's Cosplay Site!
Star Trek Original Series Uniform!
Original Creation for AB13
Most Recent Con:
PAX East, Anime Boston 2013