How to sew double sleeves on any shirt

You can put double sleeves like these on any shirt. It's easy! And this is how you do it:

 Take your favourite shirt pattern and cut the pattern piece for the arms in two. Your short sleeve will be about 2 cm longer than where you cut.

 Cut your fabric. My pattern didn't include seam allowances. Add 1 cm seam allowance on all sides of your long sleeve. Add 1 cm at the shoulder and side of the short sleeve and 5 cm where you cut the pattern.

 Sew long and short sleeve together using 1 cm seam allowance.

 Put your pattern pieces on the sleeve to see how much shorter it needs to be. The fabric in between the pattern pieces is the bit of the short sleeve overlapping the long sleeve. Fold it so that half of it comes to lie right side to right side on the long sleeve, the other half over the long sleeve.
Not sure that was clear... take the middle of the in between fabric and fold it so the two pattern pieces lie right next to each other. Put that fold onto your long sleeve.

 Stitch the short sleeve in place about 1.5 cm from the edge. You can use a twin needle or any other elastic stitch you prefer. Fold the second sleeve the same length as the first one and do the same there.

Assemble your shirt as usual.