Now, I was going to make a cape from scratch (honest) but as the only thing I could really use for fabric was so close to what the end product would be, I decided to just make a 'reconstruction' tutorial.
Now I'm sure there are other ways to make capes, but this is just what made most sense to me. There are no pockets (though you could certainly add them if you wanted) but there are little holes for your arms to go through haha.
You will need...
A sewing machine, seam ripper, scissors, one blazer / jacket / coat. The jacket I'm using is massively too big for me so I've made the shoulders smaller. You wouldn't have to do this if you used a smaller jacket, obviously. The only thing is that the length of the cape would be limited to the length of the sleeves unless you decided to add another piece on the side... but that could look weird? ANYWAY, do what you want.
Remove the sleeves using a seam ripper and set aside. Cut from the shoulders down both sides, from where you want the shoulders of your cape to sit. Either refer to your own shoulder measurements or use a jacket that fits you as a guide.
Use the seam ripper to open up the sleeves. Cut the sleeve (with the lining) into a kind of... smoother shape :|
Right sides together, sew the 'sleeve' to the front and back of the jacket, stopping where you want your little ARM SLIT (what a terrible name but 'arm hole' is kind of already taken). Do the same with the lining. After that point, sew the main fabric to the lining fabric of the same piece. So, the sleeve part will not be joined to the 'body' at that point, it will just have the lining and main fabric joined together. You know what I mean. When you want the slit to stop, rejoin the sleeve + body in the same way as before. Do the same on the other side. Now all you need to do is hem the whole thing (you should have to do some trimming first) and you are done.