Unlike making armor, sewing methods are pretty simple but very long winded.
unless you want me to write something similar to:
1, buy a sewing machine
2, buy brown canvas
3, look at the reference pictures and make a pattern that fits your body
4, straight stitch your way to victory.
you won't have much luck from me i'm afraid, teaching someone to sew is not something i ever intend to even try to tackle,
and with basic sewing knowledge you can make the coat, it's just a simple case of observing the shapes and areas where seams run along the coat, and making a paper pattern.
now if you want help on how to make the pattern, that is a different story,
but also a lengthy process,
however i can very very heavily endorse the concept that teaching a man to fish is far better than giving him a single fish.
buy this book, [link]
the costume technicians handbook, it's a pretty definitive book that teaches you how to sew, both hand and machine,
how to take precise measurements of your body or someone elses, and how to use those numbers to draft accurate, fitting patterns for costumes (i've made clothing, for myself, as everyday clothing from what i have learned from this book)
you can get it for around 10 to 15 bucks and if you read a few chapters, your costuming skills will get incredibly better, it was written by theater and hollywood professionals who make costumes as a profession, so it's really, REALLY helpful.
i know some people can be daunted by someone saying "actually all i learned was from this book" when all they want is just a tutorial, but seriously, if you want to make awesome costumes? read this book, it is priceless.