Here's how I drafted the 3/4 length sleeves and French cuffs for my self-drafted shirt.
My usual disclaimer applies that I'm no expert - just winging it from stuff off the internet, and stuff I think up! I'm putting this how-to here so I can refer back to it myself, and hopefully it will also be helpful to someone else.
Drafting the pattern pieces
This was drafted from my sleeve block which doesn't have a seam allowance, it's added at the end, and my pattern piece below includes the seam allowance. I started by deciding how far below the elbow I wanted my sleeve to finish, which is 5 cm.
Then I added 6.5 cm ease to the bottom of my sleeve (3.25 cm on each side), and ruled a line up to the underarm point to make the sleeve seam.
Next was the placket, which is the vertical line marked 9.5 cm in the photo below (I forgot to write Placket on it!). I measured the stitching line at the base of the sleeve, divided by 4, and marked the point in from the BACK edge of the sleeve. You can see the double notches to mark the back of the sleeve head on the photo above.
Then I ruled a line up at a right angle from the stitching line. This line is the 5 cm below the elbow, then 4.5 cm above the elbow - this doesn't include the seam allowance at the bottom.
The width of the cuff is the same as the width of the bottom of the sleeve, and plus 3 cm. This is for a 15 mm button extension on each short side of the cuff.
The cuff depth is 10 cm, which, when folded back on itself, makes a 5 cm cuff.
Below I've marked the 15 mm button extension, and then the 15 mm seam allowance in red. I've also notched the point of the start of the extension to line up with the edge of the sleeve during construction.
So you would cut four of these cuffs, which is what I've done in the construction photos below; but then I realised I could make each cuff from one piece of fabric, so drafted a pattern piece for that.
It's the same as the above cuff, but don't do the seam allowance at the top. Instead, extend the depth of the cuff to twice the above, and add a seam allowance at the top. So my cuff below is 20 cm deep, instead of 10 cm.
Re interfacing the cuffs, it's a bit of a judgement call as to whether to interface all or half of the cuffs. It really just depends on the fabric. I interfaced half of mine, and used Vilene G 785 interfacing. I got this from Minerva Crafts, and whilst it's £9.99 a metre, it's fabulous for viscose and slippery fabrics.
Sewing the cuffs
I will start by apologising for the photos, they were taken on my phone.
All the seam allowances are 15 mm. Mark the cut for the placket on the fabric - hopefully you can see it in green in the photo.
Cut along the line.
Make some bias tape 25 mm wide, and long enough to go around the placket.
Pin the bias tape to the WRONG SIDE of the placket, with the RIGHT SIDE of the bias tape on the placket - so right side of bias tape to wrong side of placket. Line up the raw edges.
Mark the middle of the placket with a pin - in the photo, this is the white pin third from the left.
This is what it looks like from the right side.
Sew with the needle in the ditch created from the bias tape fold. The pink tissue paper underneath is to help stablise my slippery fabric.
When you get to the middle pin, stop and pivot the sleeve around to stop it puckering.
This is what it looks like once it's sewn.
Fold the bias tape over to the right side of the sleeve, and pin in place,
then top-stitch in place. Again, stop and pivot at the centre point.
And this is what it looks like when it's finished!
Next is to attach the cuff, take one un-interfaced and one interfaced cuff.
Pin the long edge of the un-interfaced cuff to the sleeve, right sides together, lining up the notch with the edge of the placket. You can hopefully see the notch just below my finger, and the left of the white pin.
And this is what it looks like attached to the sleeve.
In the photo below, you can see the cuff extension to either side of the placket opening.
Sew this in place, starting and ending at the placket edges. Press the seam allowance up towards the cuff.
Take the interfaced cuff, and on the wrong side, press one edge up by 15mm.
Place the two cuff pieces right sides together, with the pressed up edge at the bottom. Pin the interfaced cuff to the un-interfaced cuff around the two short sides and one long side.
Make sure the seam allowance on the sleeve edge/cuff is folded up towards the cuff. And also that the pressed up edge on the interfaced cuff is also folded up - pin in place.
Sew along the three edges, pivoting at the corners. Trim, press and turn right side out. (And don't do what I did, and trim too close to the stitches!!).
The pressed edge on the interfaced cuff will still be loose. From the right side, pin this edge to the un-interfaced cuff. On the wrong side, line up the folded interfaced edge with the stitching line for the un-interfaced cuff/sleeve.
Top-stitch this in place, sewing about 1/8" in from the seam on the cuff. Sew right from the edge of the cuff, so it closes up the little gap between the edge of the cuff and the placket.
I've probably over-complicated the last few bits with my words! Hopefully it's a lot more obvious with the fabric in your hands.
The last thing to do is to make the buttonholes on the extension, there will be four buttonholes on each cuff.
I made my buttonholes 1 cm in from the edge of the cuff. Fold the cuff back on itself as in the above photo. Mark the centre of each folded edge, and that will be the centre of the buttonhole.
And that's it!