There are a few methods for drafting a mandarin collar. I am going to share my preferred method.
I began with my trusty Butterick 5678 shirt pattern that I've used to draft a number of collars and sleeves.
Since I am starting with a pattern that has been developed for the attachment of a collar, I will not need to adjust the neckline. If you are starting with a bodice that fits close to the neck, you will need to adjust the neckline. Lower the center back 1/8" to 1/4" and the shoulder at neck edge, the same amount as you lowered center back. The center front should be lowered 1/4" to 1/2". Lower the shoulder at neck edge the same amount as you did the back bodice at the shoulder. The amount that you lower the neckline depends upon how much comfort you would like around the neckline as well as the design of your garment.
I personally like a shaped mandarin collar and I like to cut it on the bias if at all possible. The only time a shaped mandarin collar will not work is if you are using a stripe fabric.
To begin, I marked the 5/8" seam allowance around the neckline of my pattern.
Begin by drawing a vertical line for center back. From the center back line, draw a horizontal line the measurement of the back neck from center back to the shoulder line. Place a notch at this point. Determine the width of your collar and draw another horizontal line from center back.
The final collar. Because the collar was cut on the bias, it sits very nicely around the neck.
Good luck and happy drafting!