A transcript from the Elna SU manual reads as follows, and the manual can be viewed on line at http://sewingonline.co.uk/instructions/elnasp/

First make a few trial runs with a scrap of material exactly as the buttonhole will be stitched, with the machine adjusted as you intend to use it.
1. Use the embroidery and buttonhole foot. It can be chnged without a screwdriver by using the fixation screw.- With the stitch width knob at "0", turn the selector to zigzag "1".
2. De-center the needle to the left.
3. Set the stitch width knob at 2, with the solid mark on top.
4. Set the stitch length dial between 1/2 and 1/4.
5. a) Mark the desired length on the material.
b) Sew the first side of the buttonhole. Stop with the needle in the material and the mark (right).
c) Raise the foot and pivot the material round the needle. Lower the foot and raise the needle.
6. a) Set the stitch width knob at 4.
b) Sew a few stitches. Raise the needle.
7. a) Set the stitch width knob at 2.
b) Sew the second side and stop just short of the length of the first. Raise the needle.
8. a) Set the stitch width knob at 4.
b) Finish off the buttonhole by sewing a few stitches. Raise the needle.
9. a) Set the stitch width knob at "0".
b) Sew a few stitches to fasten the threads, holding the material back by hand.
10. Open with the buttonhole knife, which you will find in the accessory box.