I used this tutorial from Little Kids Grow to get started with the one piece placket.
I used a collared shirt that I already had for a basic pattern. However, you can just use the shirt that you are adding the collar to as a reference for size. You want the collar to come all the way around to the edge of the placket and the collar stand to cover the placket (the stand is longer than the collar).
here we go…
1. Sew the long sides and ends of the collar, clip corners, turn, iron
2. Turn up and iron one side of collar stand 1/3 inch
3. Made a collar sandwich: un-ironed stand, collar and ironed-up stand sew
4. Open up the collar stand and pin un-ironed up side on the right side of shirt. Sew single layer of stand and shirt together
5. flip over the collar and pin the ironed edge of the stand over the seams around the top of shirt. make sure to cover the seam, then sew.