|this is a pretty idiot proof way of marking vertical buttonholes for step machines.
1. to plan placement, first 'eye' it and count the spaces, pin where top and bottom button will go
2. on a strip of paper the length between the top and bottom buttons mark divisions according to the number of spaces, either by folding the paper (even number of spaces) or measuring.
3. measure your button to determine the size of buttonhole needed, and the width from the edge.
4. Cut a strip of masking tape. Place this parallel to the opening edge, having it lined up to the inner edge of the machine foot. On my Bernina this is a short 1/8 of an inch
from the centre of the buttonhole. The tape needs to be just visible but not get caught in the stitches.
5. Mark a dot on the masking tape for the centre of each buttonhole using the paper plan, and measure out from it to get the 2 ends of the buttonhole, marking a line each end.
6. Stitching you line the inner edge of the foot up with the tape, and set the needle down parallel to the line mark, stitch down to the second mark.
its easy enough to adapt this for horizontal buttonholes. you can add a bit of tape across from your lines for more accuracy if you need it, but its not usually necessary.
hope this helps