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To begin drafting the collar pattern, you'll want to have both the front and back pattern pieces on hand along with some tracing paper and a pencil. I used a 3/8" seam allowance for this project and marked the seam allowance at the shoulder of both pattern pieces as pictured below.

You'll then want to overlap the pattern pieces at the shoulder - the seam allowance markings and the edges of the pattern pieces should line up. Tape the pieces together. This will create 1/2 of the neck opening and will give you a guide for drafting a collar. 

Place the tracing paper over the pattern pieces and start by tracing just the neck opening.

Next, make a mark about 1 inch from the left edge of the bodice front piece. This will give you a starting point for the collar. Since you'll need to fold under some fabric for the button placket and you don't want the collar itself to overlap when you button the cardigan closed, you don't want to start the collar at the very edge of the pattern piece. I folded my fabric under 1/2" to make my button plackets so that's where I came up with 1". Draw the collar the actual size you'd like it be. Keep in mind that this will create 1/2 of the collar. You'll then want to add in a 3/8" seam allowance (dotted line below) to just the bottom edge of the collar. You can now cut the collar pattern piece out and use it to cut 2 collar pieces on the fold of your fabric. You'll end up with 2 "C" shaped pieces of fabric.

If you'd like to create faux piping on your collar, use the collar pattern piece to trace out a slightly larger version...I made mine 1/8" larger. I simply traced 1/8" from the edge of the seam allowance I previously drew. You'll then cut out one standard collar piece and one slightly larger piece from a contrasting color (the larger piece will become the underside of the collar). Place the pieces right sides touching, with the sides and bottom aligned. Sew together along the bottom edge of the collar - leave the inner curve open. Turn the collar right side out and roll the bottom edge of the underside towards the front to create the look of piping. Press in place. 

If you haven't already, sew together the front, back, and sleeves of the cardigan together. Next, you'll baste the collar to the cardigan (wrong side of collar touching right side of cardigan). Center the collar on the back of the cardigan and pin in place - raw edges of the collar lined up with the raw edge of the cardigan. Baste close to the edge. You'll then want to create a strip of 1/2" wide single fold bias tape (doesn't have to be cut on the bias when using knits) the same length of the top of the cardigan. This tape will bind the seam of the collar. Open the binding and pin the right side to the collar. The top edge of the binding lines up with the raw edges of the collar/cardigan. Sew it in place using a 1/4" seam allowance.

Now you'll flip the binding in towards the inside of the cardigan. Smooth everything out and press well. Pin the binding in place and then stitch close the the edge. If you're using different color fabrics, it helps to match the bobbin thread to the fabric underneath.

The collar is finished! From this point, you can either hem or add a band to the bottom of the cardigan and finish off the sleeves. Then, fold under the front edges of the cardigan and stitch in place to create the button placket. Sew buttonholes and attach buttons....and you're done!