Start by identifying your placket and binding. The placket is the longer/wider piece and it will sit on top, the binding is the skinny piece that finishes the opening underneath. You should have a pair of each. Press the outside seam allowance of the placket under by 1cm on the top edge and 1cm on one side.
Identify your sleeve (this sleeve pattern has the front yoke included in the piece)
Turn you sleeve over so you are looking at the wrong side / inside of your sleeve.
Transfer the marking from the pattern for the slash line onto the wrong side your sleeve (ideally use a vanishing marker or chalk).
Place your placket and binding pieces onto the sleeve, either side of the slash line. A good way to remember which piece goes on which side of the slash line is - the smaller pieces goes on the smaller side of the sleeve and the bigger piece goes on the bigger side of the sleeve. Make sure you place the pieces right side down so you are looking at fusible. Transfer the corners of your slash line from the pattern onto these pieces with chalk or marker and pin in place.
Starting from the cuff edge stitch around the slash line in a box shape. The depth of the seam should be just under 1cm all the way around. 8mm either side of the slash line is a good width (so the box is 1.6cm wide in total)
Ensure the corners of your box are finished clean by pivoting the fabric with the needle down at a 90 degree angle on the corners.
Cut up the slash line through your sleeve, right up to the first marked point, then cut (very carefully) into each corner. Be careful not to cut the stitch, stop about 1mm away from the corner. If you cut too close, it will fray and you will get a hole, if you don't cut close enough you wont be able to turn the seam allowance inside cleanly.
Turn you sleeve over so you are now looking at the right side of the fabric.
Pull the binding piece(the smaller piece) through the gap so you are now looking at the right side of the sleeve and the right side of the binding. Press the seam allowance up and away creating a clean edge around your opening.
Now press the seam allowance for your binding in towards the centre of the opening and fold the opposite edge of the binding towards the seam so the edges meet. Give it a press.
Fold the binding once more, bringing the folded edge over so it sits just on top of the original stitchline. Pin it in place and Edgestitch it down. Stop the stitching just past the original corner of the box. The binding is complete.
Now pull the placket through the gap to the right side of the sleeve and give it a press to make sure the edges of the opening are clean.
Press the seam allowance into the centre of the placket. The top edge and the outside edge of your placket should already be pressed over 1cm from step 1.
Now simply fold the placket in half, bringing the edge of the placket to sit on top of the original stitch line.
Pin the placket in place - being careful to ensure it completely covers the binding underneath, also now is a good time to make sure it is an even width from top to bottom and all the seam allowance is sitting underneath the placket.
You can do this all with one row of topstitching if you start in the right place.
Start where the top of your original stitched box was (this is about 2cm from the top depending on how accurate you were with seam allowance) Its best to peek under the placket to see where this point is. Mark this place with a pin or chalk and start your topstitching there.
Continue to stitch the box around the edge of the placket, making sure to keep your needle down at the point and pivoting the fabric 90 degrees to create sharp corners. The last line will be the long edge of the placket covering the original row of stitching underneath.
Your sleeve placket is finished!
Repeat the process for the opposite sleeve - and be careful to make sure you are creating a PAIR of sleeve plackets (as shown above) It is easy to accidentally make two identical plackets but you want them to be a mirror image of each other.