The pattern I'm using has a continuous sleeve placket. It is a rectangular piece of material that is twice the length of the vent. Unfortunately the pattern gave the length for the shorter sleeve and I didn't think to check and so I had to cut out new ones. To start with, draw the cutting line on the sleeve. Then about 6mm from the bottom edge of that line reinforce the vent slash by sewing with a shorter stitch length up to the point of the slash line. Turn the material around and sew down to the edge again. Cut up to the point along the slash line without cutting through the sewing line.
Iron the placket in half lengthwise. Open it and fold one edge close to the middle and iron this fold.
Unfold the placket, spread open the sleeve vent and pin the uncreased right side to the right side of the sleeve. The middle point of the sleeve edge needs to be about 8-9mm away from the edge of the placket. (I have a piece of black thread along the edge of sleeve to show that the edges don't meet.) Sew the seam allowance distance from the edge of the placket piece. Once you get to the middle it will be tricky because there is too much material to lay flat. I tried googling how to avoid this but wasn't able to find any answer. So I just did the best I could.
Iron the seam allowance towards the placket piece. Pull the placket piece around to the wrong side of the sleeve and turn the prefolded edge under and pin down. Top stitch, being careful to hide the other seam. With right sides together, fold the placket in half and sew a short diagonal seam at the top.
Not too difficult and turned out ok for the muslin. I might try different plackets to see if I like something else better.
I used the spell check and it high-lighted almost everything! I'll try again tomorrow.