The manuals for my vintage machines all show how to make 4-step buttonholes using the zig-zag stitch. Is there anything like that in your manual?
You mark your buttonhole with a straight line that's a hair longer than the measurement of the width of the button and sew a narrow, almost satin-stitch zig-zag along the right side of the line, then turn the fabric and sew down the other side. Make sure that you don't cover the line - you will need to get in between those rows with a cutter to open the buttonhole.
Widen the zig-zag width and shorten the stitch and sew bars across the top and bottom of these two rows. Put some Fray Check on the buttonhole and let it dry, then use a sharp scissors or seam ripper to slice a slot between the rows of narrow zig-zag.