Sleeve plackets can be daunting but this is a simple method, it’s also a great way to make plackets for thick fabric or casual shirt…
Both sides sewn. 
Press a minimal seam on the edge of the placket. 
Press to the stitching. 
Sew along the outer edge. 
Press to the smaller side of the sleeve bottom. 
Pressed, ready to sew. 
Sew the under piece in place. 
Finished placket.
Lost Irishman 