|Detail of collar|
Our local craft store, Michael's, had a package of brass "rapid rivets" by Tandy Leather Factory. You can also get them at leather stores if there's one in your area. They're in assorted sized and there are just enough for the around the armholes. My doublet will use 8 on each side, but it could use 9 or 10 if you went all the way under the arm.
The photo on the right is a detail of the back of the collar, you can see the extra long piece that will be tucked under for a clean edge. If you do a long collar facing take into account the top stitching. You don't want to sew the top stitching all the way down to the edge on the back facing, stop 5/8 before the edge (the seam depth).
The top stitching sort of matches the lines on the doublet overlay. Two different views of the Flynn Rider drawing show two different match ups. I chose to go with the one that is not an exact continuation of the lines to make it easier to assemble.
The lines are farther apart in the view on the left, but in other views they seem on line with the doublet's.
Imagineers don't have to always get perfect every time. ;-)