@aplagueofzubats honestly? it was a hack job based on just eyeing it. my friend did most of the work, but the pattern i used for the coat had a straight collar pattern (as opposed to some that have curved collars). i cut the fabric for the pattern, then we folded it over at an angle to get the shape we wanted. you might want to make it a touch longer just to account for the seam allowance you probably lose from folding it over like that, but better to have it too long that not long enough.
regardless, i actually had to go in and add the lining after the fact, because the leather i’m using is not stiff enough on its own. i used bridal satin to line it, and that helped it stand up, but if you want to use a fabric actually used for lining (bridal satin is heavy as fuck), heavyweight fusible interfacing is super helpful. usually a pattern with a stiff collar calls for interfacing in general, though.
personally, i wanted it to be a bit flexible (mid summer + leather cosplay = i need to be able to fold that shit down), so i just used the bridal satin because it has the stiffness, but it’s still floppy.