Heyhey,
I reviewed the pics and noticed it were quite a few so I will split up the sleeve slit in 2 parts and put the lining in between both parts within the next week.
first I sewed the sleeves seams, the one at the sleeve slit 1cm below where the additional fabric for the slit is placed
fold the shorter end in a straight line down from the seam, put the bigger end ontop and fold it over 1cm (or what you use as seam allowance) before the edge of the lower  one
sew the seam allowances together on the top end
the both sleeves should mirror each other
also pin and sew the sleeve seams of the lining and keep the area for the slit open
pin both ends onto each other, matching the check
and cut the end even
now take each end, fold the hem allowance up and iron
pin a pin through only the top layer do mark where seam allowance and hem allowance end
fold it open, see how the pins move far away from each other?
then fold over, right side inside, pins laying on each other
mark a line from where the ironed folds cross to the pinned parking
and sew
then cut the fabric back
iron the seam allowance apart and turn around
one clean corner, repeat the same with the other corner and the corners of the other sleeve
looks pretty from the right side 😉
next time I show you how to make it look pretty inside with lining, too.
By the way, do you want a tutorial for how to add a open sleeve slit to a sleeve pattern?
If there are any questions for the steps until here please let me know!
dont forget to vote for your favorite june drawings.
keep sewing and smiling
your TailorFairy