Step 10: Cutting out the collar
To start, figure out how long your collar needs to be. I did this by holding the measuring tape at one edge of the neck opening (NOT the entire keyhole opening, but where the collar edges will join) and then measuring around the neck hole. I determined I would need roughly 20 inches, and then added 2 inches to that measurement for seem allowance. Just like with the top, it is much better to make your collar too long than too short, as we'll easily snip off the excess when we finish the collar.
Next figure out how tall your collar will be. I decided to have mine 2 inches tall, which is probably what most people will use. I then added an extra inch for seam allowance, giving the final measurement of 5 inches.
Now that you have your width and length, measure it out on your fabric. I cut mine on a fabric fold so I divided my length in half to 11 inches.
Now fold your collar with the long edges together. This is how the collar will be pinned to the top, but for the moment what we want to do is shape the collar a bit. By making the top of the collar shorter than the bottom, it will force the top of the collar to be tighter, giving the same effect as if the collar had been cut rounded (only much easier to measure, cut and sew).
Along the side with the fabric fold (which will be the top edge of our collar) mark in about 1 inch. Using the measuring tape or yard stick, angle down from the mark to the corner of the bottom edge of the collar. Mark a line and then cut. This will give you the angle needed. Repeat for other side.
Now repeat the entire process for the fabric interfacing, only making the entire thing about an inch smaller in all dimensions. When you lay the interface down on the collar, it should be at least a half inch shorter all around. Follow the instructions for your interfacing to fuse the interfacing to the wrong side of the collar.