----UPDATE----
I have uploaded the pictures of the main pattern pieces for the jacket to my scrapbook. Take a look if you are having troubles modifying your own pattern! (I couldn't find the pattern for the collar, I drew it on a piece of paper somewhere and I think it is gone now... sorry!)
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I have had a lot of requests asking how I made the jacket for my cosplay, so I decided I would write a quick overview of what went into making it. I will not go into excruciating detail, but if you would like more help on any particular section, please leave a comment and I would be happy to elaborate!

STEP 1: Picking fabric
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Print lots of good colored reference shots for yourself (or carry a smartphone with you) before heading to the fabric store. It goes without saying that you want to pick fabric with color that closely resembles your source, but it is also important to take into consideration things like the thickness and flexibility of your fabric. Conventions can get really hot really fast, so it is nice to have a fabric that is light and breathable, but I found that thin fabrics are also more challenging to work with. The inside lining for my jacket was very thin, which was great for keeping the costume cooler in the hot Arizona sun, but it had a tendency to bunch up little by little as I put it through the sewing machine. That made it difficult to align correctly with the fabric on the outside of the coat, which was considerably thicker.

So, select wisely!

STEP 2: Pick your pattern
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For me, it is much easier to make a costume if I have a pattern to work off of. However, this is a challenge in cosplay because most character outfits are unique in some way, and an accurate pattern doesn't exist. This is where you have to use your imagination and find something that may not be perfect, but bears similarities to the look you are going for.

For my Bolin jacket (and for the Mako jacket worn by ), I used this pattern. It is a pattern for a civil war costume that bears little resemblance to the outfits worn by Mako and Bolin, but there are some key features from the costume that led me to it.

First, the front of the blue jacket overlaps and connects in a way similar to Bolin's.
Second, the length of the jacket is similar to Bolin's.
Third, the jacket is gathered at the waist by a belt (and chord), much like Bolin's.

These characteristics gave me a good starting point to work from, and took a lot of the guesswork out of the problem in terms of fit and sizes. (There are lots of good tutorials online that explain how to use a pattern, so I won't be addressing that here.) Nevertheless, there are some major discrepancies between Bolin's outfit and the Civil War uniform. Which leads me to...

STEP 3: Modifying your pattern
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From looking at the pictures, you can see some obvious differences between the pattern and the jacket we want. They are:

1. The collar on the pattern is short and flat.
2. The overlapping of the skirt on the pattern doesn't line up the same way as Bolin's.
3. The jacket top and skirt are separate pieces sewn together on the pattern, whereas on Bolin's they are one.
4. The back of the civil war jacket is designed to be fitted (it is made up of multiple pieces). Bolin's jacket is not fitted, and uses the sash around his waist like a belt to fit the waistline.
5. The shoulders of the civil war jacket are a bit more broad, and the sleeves longer (and for me wider) than needed for Bolin's jacket. (Remember, the sleeves on Bolin's jacket are rolled up, so you don't need a whole lot of excess.)
6. The pants are a completely different style on this pattern than what we are looking for (so I modified a simple pajama pant pattern, which I won't go into detail on here.)

With these considerations in mind, I changed the shape of the pattern as needed. A marker, scissors, paper, and tape are your friends here. Draw extra shape where it is needed, tape together pieces that should be connected, and use your reference pictures to figure out how everything should fit together. It helps to visualize how your new pieces will look by holding them up on your body. If you are fortunate enough to have a dress form the process is even easier. (I should really get one of those...)

It is too hard to describe exactly what changes I made to the pattern, so here are the pictures of the final pieces for the jacket!

Jacket Front: [link] - Cut 2 of these
Jacket Back: [link] - Cut on the fold to make 1 big piece for the back
Sleeves: [link] - Cut 2 of these, obviously

I'm sorry I couldn't find my pattern piece for the collar, but if you have been able to make any sense of what I have said so far, you should be able to figure it out! I believe in you!

STEP 4: Sew it all together
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I will not go into much detail here because my assumption is you know how to work a sewing machine and sew together pieces of a pattern. Again, there are lots of good tutorials for this online if you need help. Or even better, get someone experienced to teach you how this works (thank you Mom!).

Depending on your fabric, you may need interfacing to help the jacket keep its shape and not droop while it is on you. Keep this in mind before you sew everything up!

STEP 5: Add the details
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Add the edging of the coat by cutting strips out of the same fabric used for the jacket lining and attaching where necessary. Alternatively, you can buy pre-pressed strips of fabric used for quilt edging (so long as the color matches). I did this for the edging on the under-shirt and it saved me some time.

Make the little rectangle dealies for closing the coat and connect them however you like - snaps, hooks, or even velcro work great.

STEP 6: The rest and enjoy
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Put whatever finishing touches on that you think is appropriate. I am sure I forgot something and I know I didn't cover everything. I'm very sorry!

I hope this has helped you in some way, shape, or form! If you have more questions PLEASE feel free to ask and I would be happy to respond. Thanks!