Create sleeve pattern:

Put on T-shirt and mark in the desired curve at the sleeve. Indicate the point of the shoulder seam on the sleeve section. The sleeve serves as a template for the tulle sleeve you will be constructing.

Cut off the sleeve and cut through it along the marked line.

Unpick the sleeve seam. Place the resultant piece on a sheet of paper as a template and trace around it. Now continue working on the paper pattern. Mark shoulder point at top of pattern and indicate sleeve centre on bottom edge. Join points with a vertical line.

Subdivide sleeve pattern: draw in 5 vertical lines as in the sketch.

Cut sleeve pattern apart at marked lines.

Rearrange the pieces and glue onto a new piece of paper the sketched distances apart. Adjust curves at top and bottom and re-label centre point. Cut out the pattern piece thereby created (= lower-sleeve section).
 
For the upper sleeve, copy the template and shorten by 3.5 cm at the bottom edge, tapering off the curve at the side seam to the existing size. This yields the upper-sleeve piece.
 
Cutting out tulle fabric:
 
2 x upper sleeve
4 x lower sleeve
 
 
 
Instructions:
 
For each sleeve, pin together 2 long and one short sleeve section. Close sleeve seam. Gather top curve up to 10 cm before and after sleeve seam. Turn sleeve. 

Neaten the sleeve curve on the T-shirt. Pin or baste tulle sleeves all around onto the right side of the shirt and sew with a small zigzag stitch. 

Sew frills onto the sleeve curve and bottom edge of the tulle as desired.